Fix #790: Don't emit NV-specific interface members unless enabled by extension.

Test/spv.450.noRedecl.tesc

@@ -0,0 +1,10 @@
+#version 450 core
+
+layout(vertices = 4) out;
+
+patch out vec4 patchOut;
+
+void main()
+{
+    gl_in[0].gl_PointSize;
+}