Out-of-tree build fails with uverr2result.c:15:10: fatal error: isc/platform.h: No such file or directory

Out-of-tree build fails with:

libtool: compile:  gcc -include /builds/isc-projects/bind9/workspace/config.h -I/builds/isc-projects/bind9/workspace -I../../../.. -I../../../../lib/isc/netmgr/../include -I../../../../lib/isc/netmgr/../unix/include -I../../../../lib/isc/netmgr/../pthreads/include -I../../../../lib/isc/netmgr/.. -I/usr/include/json-c -I/usr/include/libxml2 -DISC_MEM_DEFAULTFILL=1 -DISC_LIST_CHECKINIT=1 -fno-omit-frame-pointer -fno-optimize-sibling-calls -O1 -g -Wall -Wextra -O3 -pthread -I/usr/include/google -fPIC -W -Wall -Wmissing-prototypes -Wcast-qual -Wwrite-strings -Wformat -Wpointer-arith -Wno-missing-field-initializers -fno-strict-aliasing -Wshadow -Werror -c ../../../../lib/isc/netmgr/tcp.c  -fPIC -DPIC -o .libs/tcp.o
../../../../lib/isc/netmgr/uverr2result.c:15:10: fatal error: isc/platform.h: No such file or directory
   15 | #include <isc/platform.h>
      |          ^~~~~~~~~~~~~~~~
compilation terminated.
make[3]: *** [Makefile:185: uverr2result.lo] Error 1
In file included from ../../../../lib/isc/netmgr/../include/isc/buffer.h:105,
                 from ../../../../lib/isc/netmgr/udp.c:16:
../../../../lib/isc/netmgr/../include/isc/assertions.h:20:10: fatal error: isc/platform.h: No such file or directory
   20 | #include <isc/platform.h>
      |          ^~~~~~~~~~~~~~~~
compilation terminated.
make[3]: *** [Makefile:185: udp.lo] Error 1
In file included from ../../../../lib/isc/netmgr/../include/isc/buffer.h:105,
                 from ../../../../lib/isc/netmgr/tcpdns.c:16:
../../../../lib/isc/netmgr/../include/isc/assertions.h:20:10: fatal error: isc/platform.h: No such file or directory
   20 | #include <isc/platform.h>
      |          ^~~~~~~~~~~~~~~~
compilation terminated.
In file included from ../../../../lib/isc/netmgr/../include/isc/buffer.h:105,
                 from ../../../../lib/isc/netmgr/netmgr.c:17:
../../../../lib/isc/netmgr/../include/isc/assertions.h:20:10: fatal error: isc/platform.h: No such file or directory
   20 | #include <isc/platform.h>
      |          ^~~~~~~~~~~~~~~~
compilation terminated.
In file included from ../../../../lib/isc/netmgr/../include/isc/buffer.h:105,
                 from ../../../../lib/isc/netmgr/tcp.c:17:
../../../../lib/isc/netmgr/../include/isc/assertions.h:20:10: fatal error: isc/platform.h: No such file or directory
   20 | #include <isc/platform.h>
      |          ^~~~~~~~~~~~~~~~
compilation terminated.
make[3]: *** [Makefile:185: tcpdns.lo] Error 1
make[3]: *** [Makefile:185: tcp.lo] Error 1
make[3]: *** [Makefile:185: netmgr.lo] Error 1
make[3]: Target 'all' not remade because of errors.
make[3]: Leaving directory '/builds/isc-projects/bind9/workspace/lib/isc/netmgr'
make[2]: *** [Makefile:212: subdirs] Error 1

We had a similar one recently in 9.14 branch: #1530 (closed).