glsl/linker: Fail to link geometry shader without vertex shader.

glsl/linker: Fail to link geometry shader without vertex shader.

From section 2.15 (Geometry Shaders) the OpenGL 3.2 spec:

    A program object that includes a geometry shader must also include
    a vertex shader; otherwise a link error will occur.

Reviewed-by: Paul Berry <stereotype441@gmail.com>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>