fix incorrect error when multiple compilation units don't declare layouts (#2238)

when using multiple compilation units, input/output layouts don't need
to be declared in every unit.

Test/contradict_1.geom

@@ -0,0 +1,9 @@
+#version 330
+
+layout(lines) in;
+layout(line_strip, max_vertices = 6) out;
+
+float getV()
+{
+    return 1.0;
+}