glsl: only validate xfb_buffer values when we have enhanced layouts

glsl: only validate xfb_buffer values when we have enhanced layouts

XFB might not be supported, and the shader wouldn't be setting this
flag. But validation would still fail, since the number of xfb buffers
would be 0. So only validate if an xfb_buffer is set in the qualifiers.

See: https://gitlab.freedesktop.org/mesa/mesa/-/issues/5415
Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>
Reviewed-by: Timothy Arceri <tarceri@itsqueeze.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/14869>