package/xscreensaver: add gl dependency

Fix the following build failure without gl raised since bump to version
6.03 in commit cbcbd66b:

       Note: The OpenGL 3D library is not being used.

             Those demos which use 3D will not be built or installed.
             That is most of them.

make: *** [package/pkg-generic.mk:282: /home/peko/autobuild/instance-0/output-1/build/xscreensaver-6.03/.stamp_configured] Error 1

A patch was sent to Jamie Zawinski to fix this failure and he replied:

"Why would you do that? I pretty much consider that an unsupported
configuration, since that's more than half of the savers at this point."

I told him that some embedded devices don't have gl and he replied:

"Such as? Just about every modern "embedded device" I'm aware of is more
performant than the desktop computers on which I developed many of those
GL hacks..."

So just add a dependency on gl.

Fixes:
 - http://autobuild.buildroot.org/results/b76743a48d75b5b7539712318fa0b2937a157f88



Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
[yann.morin.1998@free.fr: keep --with-gl=yes]
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>