C prototypes for functions returning C function pointers.

C prototypes for functions returning C function pointers.

This patch handles dumping prototypes for C functions returning
function pointers.  For the test case

MODULE test
   USE, INTRINSIC :: ISO_C_BINDING
CONTAINS
   FUNCTION lookup(idx) BIND(C)
     type(C_FUNPTR) :: lookup
     integer(C_INT), VALUE :: idx
     lookup = C_FUNLOC(x1)
   END FUNCTION lookup

   subroutine x1()
   end subroutine x1
 END MODULE test

the prototype is

void (*lookup (int idx)) ();

Regression-tested.  Again no test case because I don't know how.
During testing, I also found that vtabs were dumped, this is
also corrected.

gcc/fortran/ChangeLog:

	PR fortran/119419
	* dump-parse-tree.cc (write_funptr_fcn): New function.
	(write_type): Invoke it for C_FUNPTR.
	(write_interop_decl): Do not dump vtabs.