egl: Do not export private symbols

egl: Do not export private symbols

libEGL was incorrectly exporting *all* symbols, public and private.
This patch adds -fvisibility=hidden to libEGL's linker flags to ensure
that only symbols annotated with __attribute__((visibility("default")))
get exported.

Sanity-checked with libEGL's builtin DRI2 driver and the i965 DRI driver
by running Piglit on X/EGL and by running weston-gears on Weston as an
X client.

Sanity-checked with libEGL's Gallium driver (which is not built-in) and
the swrast Gallium driver by running es2gears_x11.

Kristian reviewed the symbol diff in `nm libEGL.so`.

CC: "9.2" <mesa-stable@lists.freedesktop.org>
CC: Ian Romanick <idr@freedesktop.org>
Acked-by: Kristian Høgsberg <krh@bitplanet.net>
Reviewed-by: Jakob Bornecrantz <jakob@vmware.com>
Signed-off-by: Chad Versace <chad.versace@linux.intel.com>