mesa/main: only call _mesa_destroy_shader_compiler once on exit

mesa/main: only call _mesa_destroy_shader_compiler once on exit

There's no point in calling _mesa_destroy_shader_compiler multiple
times on exit; the resources will only be released once anyway.

So let's move the atexit-call into the part that is only called
once.

Signed-off-by: Erik Faye-Lund <kusmabite@gmail.com>
Reviewed-by: Matt Turner <mattst88@gmail.com>
Reviewed-by: Brian Paul <brianp@vmware.com>