headers_check: don't warn about c++ guards

A recent addition to the DRM tree for 4.7 added 'extern "C"' guards
for c++ to all the DRM headers, and that now causes warnings
in 'make headers_check':

usr/include/drm/amdgpu_drm.h:38: userspace cannot reference function or variable defined in the kernel
usr/include/drm/drm.h:63: userspace cannot reference function or variable defined in the kernel
usr/include/drm/drm.h:699: userspace cannot reference function or variable defined in the kernel
usr/include/drm/drm_fourcc.h:30: userspace cannot reference function or variable defined in the kernel
usr/include/drm/drm_mode.h:33: userspace cannot reference function or variable defined in the kernel
usr/include/drm/drm_sarea.h:38: userspace cannot reference function or variable defined in the kernel
usr/include/drm/exynos_drm.h:21: userspace cannot reference function or variable defined in the kernel
usr/include/drm/i810_drm.h:7: userspace cannot reference function or variable defined in the kernel

This changes the headers_check.pl script to not warn about this.
I'm listing the merge commit as introducing the problem, because
there are several patches in this branch that each do this for
one file.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Fixes: 7c10ddf87472 ("Merge branch 'drm-uapi-extern-c-fixes' of https://github.com/evelikov/linux into drm-next")
Reviewed-by: Emil Velikov <emil.l.velikov@gmail.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>