glsl: Fix cross-version linking between VS and GS.

Previously, when attempting to link a vertex shader and a geometry
shader that use different GLSL versions, we would sometimes generate a
link error due to the implicit declaration of gl_PerVertex being
different between the two GLSL versions.

This patch fixes that problem by only requiring interface block
definitions to match when they are explicitly declared.

Fixes piglit test "shaders/version-mixing vs-gs".