De-duplicate '#define _GNU_SOURCE' in 'libgomp/config/linux/allocator.c'

Minor fix-up for commit 348874f0baac0f22c98ab11abbfa65fd172f6bdd
"libgomp: basic pinned memory on Linux".

	libgomp/
	* config/linux/allocator.c: De-duplicate '#define _GNU_SOURCE'.
1 file changed