Add fmgr.h include to selfuncs.h.

Necessary after fb3b098f. That previously escaped notice, because all
including sites already include fmgr.h some other way.

Reported-By: Tom Lane
Author: Andres Freund
Discussion: https://postgr.es/m/17463.1566153454@sss.pgh.pa.us

src/include/utils/selfuncs.h

@@ -16,6 +16,7 @@
 #define SELFUNCS_H
 
 #include "access/htup.h"
+#include "fmgr.h"
 #include "nodes/pathnodes.h"