testsuite, Objective-C : Fix call-super-2.m for newer NeXT headers.

testsuite, Objective-C : Fix call-super-2.m for newer NeXT headers.

We were using a callout to runtime.h which, eventually brings in the system
runtime.h.  One newer versions of the Darwin/NeXT headers this declares the
objc_getClass() function as returning Class, rather than the internal
representation of that.  This breaks a fragile assumption in objc-act that
the use of these internal functions can be deduced by looking at the function
type.  Ultimately, the fragility should be fixed - but this fixes up the test
so that it only uses the compiler's forward declaration of the function.

gcc/testsuite/ChangeLog:

	* objc.dg/call-super-2.m: Remove inclusion of runtime.h.
	Add a FIXME about the test portability.