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>

src/egl/egl-glvnd-symbols.txt

@@ -0,0 +1 @@
+__egl_Main

src/egl/meson.build

@@ -194,7 +194,7 @@ pkg.generate(
 
 if with_tests and prog_nm.found()
   if with_glvnd
-    # TODO: add glvnd symbol check
+    egl_symbols = files('egl-glvnd-symbols.txt')
   else
     egl_symbols = files('egl-symbols.txt')
   endif