egl: hide entrypoints that shouldn't be exported when using glvnd

From GLVND author @kbrenneman (!157 (merged)):

From a functional standpoint, exporting additional symbols doesn't really matter, since libglvnd will load the vendor libraries with RTLD_LOCAL.

Adding a version script for compilers that support it to limit the exported symbols to __egl_Main wouldn't hurt. For other systems, though, I'd say it's probably not worth the hassle.

That's the solution I went with, hiding the symbols when possible.