_G_config.h: don't include <cstddef>

<stddef.h> is just fine and it doesn't break the use of libroot headers
when compiling something with -nostdinc++ (in C++ mode).
Not particularly important, but this gets compiling the libio C++ stuff
when building a native gcc a bit further. It still fails, since our
<printf.h> header is actually not usable (it includes <features.h>,
which is not available in Haiku) -- something we should fix eventually.

headers/glibc/_G_config.h

@@ -23,11 +23,7 @@
 #define __need_wint_t
 #define __need_NULL
 #define __need_ptrdiff_t
-#ifdef __cplusplus
-#	include <cstddef>
-#else
-#	include <stddef.h>
-#endif
+#include <stddef.h>
 
 //#include <wchar.h>