meson: expose glapi through osmesa

Suggested-by: Pierre Guillou <pierre.guillou@lip6.fr>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=109659


Fixes: f121a669 "meson: build gallium based osmesa"
Fixes: cbbd5bb8 "meson: build classic osmesa"
Cc: Brian Paul <brianp@vmware.com>
Cc: Dylan Baker <dylan@pnwbakers.com>
Signed-off-by: Eric Engestrom <eric.engestrom@intel.com>
Tested-by: Chuck Atkins <chuck.atkins@kitware.com>
(cherry picked from commit ccb8ea7a)