pgsql: Some of our port-specific dynloader implementations are careful

Log Message:
-----------
Some of our port-specific dynloader implementations are careful to
define pg_dlsym() as returning a PGFunction pointer, not just any
pointer-to-function.  But many are not.  Suppress compiler warnings
on platforms that aren't careful by inserting explicit casts at the
two call sites that didn't have a cast already.  Per Stefan.

Modified Files:
--------------
    pgsql/src/backend/utils/fmgr:
        dfmgr.c (r1.94 -> r1.95)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/fmgr/dfmgr.c.diff?r1=1.94&r2=1.95)