util: compile drm.o on posix

OpenGL isn't required to use DRM rendernodes. The following patches
uses it for 2d resources for ex.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-id: 20190524130946.31736-5-marcandre.lureau@redhat.com

[ kraxel s/LINUX/POSIX/ (fixes openbsd build failure) ]

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>