Put back mistakenly removed #include.
In commit I removed some code from parse_coerce.c, and also
removed some apparently-no-longer-needed #includes.  But removing
datum.h broke some not-compiled-by-default code.
Discussion:
#include "parser/parse_relation.h"
#include "parser/parse_type.h"
#include "utils/builtins.h"
+#include "utils/datum.h" /* needed for datumIsEqual() */
#include "utils/lsyscache.h"
#include "utils/syscache.h"
#include "utils/typcache.h"
