Fix string/bug-strncat1.c build with GCC 8.

Fix string/bug-strncat1.c build with GCC 8.

GCC 8 warns about strncat calls with truncated output.
string/bug-strncat1.c tests such a call; this patch disables the
warning for it.

Tested (compilation) with GCC 8 for x86_64-linux-gnu with
build-many-glibcs.py (in conjunction with Martin's patch to allow
glibc to build).

	* string/bug-strncat1.c: Include <libc-diag.h>.
	(main): Disable -Wstringop-truncation for strncat call for GCC 8.