Revert "mesa: Remove GLXContextID typedef from glx.h."

This reverts commit 136a12ac98.

According to belak51 on IRC, this commit broke Allegro, which would no
longer compile.  Applications apparently expect the GLXContextID typedef
to exist in glx.h; removing it breaks them.  A bit of searching around
the internet revealed other complaints since upgrading to Mesa 10.

Cc: "10.0" <mesa-stable@lists.freedesktop.org>

include/GL/glx.h

@@ -168,6 +168,7 @@ typedef XID GLXDrawable;
 /* GLX 1.3 and later */
 typedef struct __GLXFBConfigRec *GLXFBConfig;
 typedef XID GLXFBConfigID;
+typedef XID GLXContextID;
 typedef XID GLXWindow;
 typedef XID GLXPbuffer;