Include <stdlib.h> in lib/dns/dnsrps.c

Since commit 0771dd3b, <isc/mem.h> no longer includes <isc/xml.h>. On some systems (e.g. FreeBSD), this means that no header included by lib/dns/dnsrps.c (and no header included by those headers) contains a definition of free() any more, which triggers a compiler warning as lib/dns/dnsrps.c calls that function. Add the missing #include directive to prevent that warning from being triggered.

Closes #1108 (closed)