tools/Makefile: Move _GNU_SOURCE to Makefile

Commit 669dfc2e adds libfdt_env.h to HOSTCPPFLAGS. It causes stdio.h
to be included before _GNU_SOURCE is defined in C files. On some old hosts
some prototypes are protected by #ifdef __USE_GNU, which is set when
_GNU_SOURCE is defined.

Signed-off-by: York Sun <yorksun@freescale.com>
Acked-by: Simon Glass <sjg@chromium.org>