gs: Don't try to use gl_PerVertex when redeclaring fragment shader inputs.

In commit 5b1dff3 (Geometry shaders: Test gl_ClipDistance input (GLSL
1.50)) I accidentally used this syntax to redeclare the
gl_ClipDistance fragment shader input:

    in gl_PerVertex {
      float gl_ClipDistance[8];
    };

This is incorrect--gl_PerVertex is only meaningful in vertex and
geometry shaders.  In fragment shaders gl_ClipDistance isn't in any
interface block.