libgomp.texi (OpenMP TR11 impl. status): Fix 'strict' item

Fix the 'strict' modifier status: it is already listed (as 'Y') for OpenMP
5.1 for num_task and grainsize; only strict on num_threads is new with TR11.

libgomp/
	* libgomp.texi (OpenMP TR11): Fix item for 'strict' modifier.