meson: Add dep_glvnd to egl deps when building with glvnd

Otherwise if glvnd is not installed systemwide, but only in a prefix,
it's headers wont be found. This happens because if it's headers are in
/usr/include/ then another dependence will provide the necessary -I
arguments and compilation will work.

Fixes: 035ec7a2
       ("meson: Add support for EGL glvnd")
Acked-by: Eric Engestrom <eric@engestrom.ch>
(cherry picked from commit 5d085ad0)