configure.ac: bump libdrm requirement to 2.4.68

Had a stale libdrm sitting around which gave me errors, both fixed with the
.68 version.

libweston/pixel-formats.c:291:13: error: ‘DRM_FORMAT_NV24’ undeclared here
(not in a function); did you mean ‘DRM_FORMAT_NV21’?
   .format = DRM_FORMAT_NV24,
             ^~~~~~~~~~~~~~~
             DRM_FORMAT_NV21
libweston/pixel-formats.c:296:13: error: ‘DRM_FORMAT_NV42’ undeclared here
(not in a function); did you mean ‘DRM_FORMAT_NV12’?
   .format = DRM_FORMAT_NV42,
             ^~~~~~~~~~~~~~~
             DRM_FORMAT_NV12

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>
Acked-by: Daniel Stone <daniels@collabora.com>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>
Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>