egl: fix build race in automake

egl: fix build race in automake

There is a parallel make build issue in src/egl/drivers/dri2/
for wayland builds. Can be reproduced with:

$ rm src/egl/drivers/dri2/*.h src/egl/drivers/dri2/platform_wayland.lo
$ make -C src/egl/ drivers/dri2/platform_wayland.lo
../../../mesa-18.1.2/src/egl/drivers/dri2/platform_wayland.c:50:10: fatal error: linux-dmabuf-unstable-v1-client-protocol.h: No such file or directory

This patch adds the missing dependency.

Fixes: 02cc359372773800de817 "egl/wayland: Use linux-dmabuf interface for buffers"
Reviewed-by: Eric Engestrom <eric.engestrom@intel.com>
[Eric: fixed up the commit title]
Signed-off-by: Eric Engestrom <eric.engestrom@intel.com>