Changes to src/lib/dhcpsrv/alloc_engine_messages.mes they are never picked up during build,

Per support ticket https://support.isc.org/Ticket/Display.html?id=15006

while testing out kea-1.6.0-beta2, I found a minor typo in a Makefile.

When doing changes to src/lib/dhcpsrv/alloc_engine_messages.mes they are never picked up during build, as make is generating some other files if changes are detected.

Attached patch that should fix the problem.

diff --git a/src/lib/dhcpsrv/Makefile.am b/src/lib/dhcpsrv/Makefile.am
index cb022d7e3e..1177ac62af 100644
--- a/src/lib/dhcpsrv/Makefile.am
+++ b/src/lib/dhcpsrv/Makefile.am
@@ -255,7 +255,7 @@ messages: alloc_engine_messages.h alloc_engine_messages.cc \
        @echo Message files regenerated
 
 alloc_engine_messages.h alloc_engine_messages.cc: alloc_engine_messages.mes
-       $(top_builddir)/src/lib/log/compiler/kea-msg-compiler $(top_srcdir)/src/lib/dhcpsrv/dhcpsrv_messages.mes
+       $(top_builddir)/src/lib/log/compiler/kea-msg-compiler $(top_srcdir)/src/lib/dhcpsrv/alloc_engine_messages.mes
 
 dhcpsrv_messages.h dhcpsrv_messages.cc: dhcpsrv_messages.mes
        $(top_builddir)/src/lib/log/compiler/kea-msg-compiler $(top_srcdir)/src/lib/dhcpsrv/dhcpsrv_messages.mes