glsl: Apply the link error conditions to GL_ARB_fragment_coord_conventions

glsl: Apply the link error conditions to GL_ARB_fragment_coord_conventions

Link error conditions added in previous patch are equally applicable
to GL_ARB_fragment_coord_conventions implementation. Extension's spec
says:
   "If gl_FragCoord is redeclared in any fragment shader in a program,
    it must be redeclared in all the fragment shaders in that program
    that have a static use of gl_FragCoord. All redeclarations of
    gl_FragCoord in all fragment shaders in a single program must have
    the same set of qualifiers."

Signed-off-by: Anuj Phogat <anuj.phogat@gmail.com>
Cc: <mesa-stable@lists.freedesktop.org>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>