glx: Fix build errors with --enable-mangling (v2)

glx: Fix build errors with --enable-mangling (v2)

Rearranged the GLX_ALIAS macro in glextensions.h so that it will pick up
the renames from glx_mangle.h.

Fixed the alias attribute for glXGetProcAddress when USE_MGL_NAMESPACE is
defined.

v2: Add a comment clarifying why GLX_ALIAS needs two macros.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=55552
Signed-off-by: Kyle Brenneman <kbrenneman@nvidia.com>
Cc: "10.6 11.0" <mesa-stable@lists.freedesktop.org>
Reviewed-by: Emil Velikov <emil.l.velikov@gmail.com>