mesa3d: add missing select of xproto_glproto

When X.org is available, mesa3d has xproto_glproto in its
dependencies, but it forgets to select it. This commit fixes this
mistake, which fixes the following autobuilder failure:

  http://autobuild.buildroot.org/results/378/378bbf8520d67a67b5906ec177079542158a6be7/



Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>