meson: move gl_common to gl_generic

meson: move gl_common to gl_generic

Summary:
sooo, gl_common stuff is only used by gl_generic, (and maybe others.
However, it is build static anyways and only linked into gl_generic).

This means, only software_generic is still in here and we can remove one
special case from the build instructions.

Reviewers: zmike, stefan_schmidt, cedric, vtorri

Reviewed By: zmike

Subscribers: #reviewers, #committers

Tags: #efl

Differential Revision: https://phab.enlightenment.org/D8667

Change-Id: Id7b2641c95d7581a43da4a07dc8531347f8b6caf