meson: Don't link gbm with threads

It's supposed to be linked with pthread-stubs (if the platform needs
pthread-stubs). Pthread stubs support isn't (yet) implemented in the
meson build, so add a TODO.

Signed-off-by: Dylan Baker <dylanx.c.baker@intel.com>
Reviewed-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Eric Engestrom <eric.engestrom@imgtec.com>

src/gbm/meson.build

@@ -34,7 +34,7 @@ deps_gbm = []
 
 if with_dri2
   files_gbm += files('backends/dri/gbm_dri.c', 'backends/dri/gbm_driint.h')
-  deps_gbm += [dep_libdrm, dep_thread]
+  deps_gbm += dep_libdrm # TODO: pthread-stubs
   args_gbm += '-DDEFAULT_DRIVER_DIR="@0@"'.format(dri_driver_dir)
 endif
 if with_platform_wayland