graphics/imv: allow building on Wayland-only
imv can work on a system where libX11 is fully removed
and OpenGL is provided by libOpenGL instead of libGL[1].
Adjust USE_GL for imv to build in such environment.
[1]: https://github.com/eXeC64/imv/commit/
a70727b74c532ffb303023b3eb215310948b31
Rebased version is always available here:
https://codeberg.org/ei/ports/commits/branch/ei