Ensure guc_tables.o's dependency on guc_tables.inc.c is known.

Without this, rebuilds can malfunction unless --enable-depend is used.
Historically we've expected that you can get away without
--enable-depend as long as you manually clean after changing *.h
files; the makefiles are supposed to handle other sorts of
dependencies.  So add this one.

Follow-on to 635998965, so no need for back-patch.

Discussion: https://postgr.es/m/3121329.1758650878@sss.pgh.pa.us

src/backend/utils/misc/Makefile

@@ -40,6 +40,9 @@ ifdef krb_srvtab
 override CPPFLAGS += -DPG_KRB_SRVTAB='"$(krb_srvtab)"'
 endif
 
+# Force this dependency to be known even without dependency info built:
+guc_tables.o: guc_tables.c $(top_builddir)/src/backend/utils/guc_tables.inc.c
+
 include $(top_srcdir)/src/backend/common.mk
 
 clean: