egl: add glvnd symbols check

egl: add glvnd symbols check

According to the spec [1], `__egl_Main` is the only symbol that needs to
be exported. We don't want applications directly linking against
libEGL_mesa.so (apps should always go through libEGL.so, regardless of
who is providing it), so we shouldn't export any other symbols either.

[1] https://github.com/NVIDIA/libglvnd/blob/master/include/glvnd/libeglabi.h
    (this header is the closest there is to a spec)

Signed-off-by: Eric Engestrom <eric.engestrom@intel.com>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>