x11-toolkits/wlroots: Add mesa-dri to RUN_DEPEND

wlroots will try to create a GBM device and this need the gbm drivers from
mesa.

Fixes running any wayland compositor depending on wlroots on a fresh install.
Example:
$ pkg install sway alacritty
$ sway
gbm: failed to open any driver (search paths /usr/local/lib/dri)
gbm: Last dlopen error: Cannot open "/usr/local/lib/dri/i965_dri.so"
failed to load driver: i965
gbm: failed to open any driver (search paths /usr/local/lib/dri)
gbm: Last dlopen error: Cannot open "/usr/local/lib/dri/kms_swrast_dri.so"
failed to load driver: kms_swrast
gbm: failed to open any driver (search paths /usr/local/lib/dri)
gbm: Last dlopen error: Cannot open "/usr/local/lib/dri/swrast_dri.so"
failed to load swrast driver
2020-02-28 12:21:31 - [backend/drm/renderer.c:19] Failed to create GBM device
2020-02-28 12:21:31 - [backend/drm/backend.c:203] Failed to initialize renderer
2020-02-28 12:21:31 - [backend/backend.c:163] Failed to open DRM device 12
2020-02-28 12:21:31 - [backend/backend.c:304] Failed to open any DRM device
2020-02-28 12:21:31 - [sway/server.c:47] Unable to create backend

Approved by: jbeich (maintainer)
Differential Revision: https://reviews.freebsd.org/D23853