testsuite,objective-c++: Fix imported NSObjCRuntime.h.

testsuite,objective-c++: Fix imported NSObjCRuntime.h.

We have imported some headers from the GNUStep project to allow us
to maintain the testsuite independent to changing versions of system
headers.

One of these headers has a macro that (now we have support for
__has_feature) expands to a declaration that triggers a warning.

These headers are considered part of the implementation so that, in
this case, we can suppress the warning with the system_header pragma.

Signed-off-by: Iain Sandoe <iain@sandoe.co.uk>
gcc/testsuite/ChangeLog:

	* objc-obj-c++-shared/GNUStep/Foundation/NSObjCRuntime.h: Make
	this header use pragma system_header.