package/mesa3d: Add xcb-fixes to loader when using x11 and dri3

"loader_dr3_helper.c uses xcb_xfixes_create_region() that requires dep_xcb_xfixes to link.
This is dependent on with_platform_x11 and with_dri3.
But the source meson file does not set this up dependent on with_dri3."

i686-buildroot-linux-gnu/bin/ld: src/loader/libloader_dri3_helper.a(loader_dri3_helper.c.o): in function `loader_dri3_swap_buffers_msc':
loader_dri3_helper.c:(.text.loader_dri3_swap_buffers_msc+0x33e): undefined reference to `xcb_xfixes_create_region'

Fixes:
https://gitlab.com/buildroot.org/buildroot/-/jobs/830981830



Signed-off-by: Romain Naour <romain.naour@gmail.com>
Cc: Bernd Kuhls <bernd.kuhls@t-online.de>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>