pgsql: Avoid bare 'struct Node;' declaration --- provokes annoying

Log Message:
-----------
Avoid bare 'struct Node;' declaration --- provokes annoying warnings
on some compilers.

Modified Files:
--------------
pgsql/src/include:
fmgr.h (r1.38 -> r1.39)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/fmgr.h.diff?r1=1.38&r2=1.39)