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

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

Since commit 0771dd3be8bad18f669de978f3be5e08cf2dbd6e, <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.