Put back mistakenly removed #include.

In commit 4dbcb3f8 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: https://postgr.es/m/20200407205436.pyjhddw5bn5upvsu@development

src/backend/parser/parse_coerce.c

@@ -25,6 +25,7 @@
 #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"