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

Since commit 0771dd3be8, <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.

lib/dns/dnsrps.c

@@ -16,6 +16,8 @@
 
 #ifdef USE_DNSRPS
 
+#include <stdlib.h>
+
 #include <isc/mem.h>
 #include <isc/string.h>
 #include <isc/util.h>