meson: rename `glvnd_missing_pc_files` to `not glvnd_has_headers_and_pc_files`

This reflects better what is provided by glvnd or not.

Fixes: 93df862b ("meson: re-add incorrect pkg-config files with GLVND for backward compatibility")
Signed-off-by: Eric Engestrom <eric.engestrom@intel.com>
Reviewed-by: Dylan Baker <dylan@pnwbakers.com>
(cherry picked from commit b57fa7ca)