Fix incorrect link time validation for unused gl_PerVertex members

Test/link.redeclareBuiltin.geom

@@ -0,0 +1,23 @@
+#version 410
+
+layout(triangles) in;
+layout(triangle_strip, max_vertices=3) out;
+
+in gl_PerVertex
+{
+  vec4 gl_Position;
+} gl_in[];
+
+void main()
+{
+    gl_Position = gl_in[0].gl_Position;
+    EmitVertex();
+
+    gl_Position = gl_in[1].gl_Position;
+    EmitVertex();
+
+    gl_Position = gl_in[2].gl_Position;
+    EmitVertex();
+
+    EndPrimitive();
+}