egl/main: no longer export internal function

egl/main: no longer export internal function

With the split of the gallium egl module we had previously it required
access to some of the internal functions. As the only build (automake)
that did this no longer builds it we can now appropriately hide those
functions.

Cc: 10.5 <mesa-stable@lists.freedesktop.org>
Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>
Reviewed-by: Matt Turner <mattst88@gmail.com>
Reviewed-by: Brian Paul <brianp@vmware.com>