dri/swrast: automake: add LIBDRM_CFLAGS

dri/swrast: automake: add LIBDRM_CFLAGS

With the follow up commit we'll remove the __NOT_HAVE_DRM_H macro. As
requested by Ian HAVE_LIBDRM will be used instead, which will lead to
swrast including drm.h when libdrm package is available, even though we
don't need/make use of the header.

As the define is added after the AM_CFLAGS we cannnot use -UHAVE_LIBDRM,
but instead let's just add LIBDRM_CFLAGS. The latter of which will
expand to NULL when the libdrm package is not around.

Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>