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

Test/spv.450.geom

@@ -0,0 +1,20 @@
+#version 450 core
+
+layout(triangles) in;
+
+in gl_PerVertex {
+    float gl_PointSize;
+} gl_in[];
+
+out gl_PerVertex {
+    float gl_PointSize;
+};
+
+layout(line_strip) out;
+layout(max_vertices = 127) out;
+layout(invocations = 4) in;
+
+void main()
+{
+    float p = gl_in[1].gl_PointSize;
+}