Allow structs with no members.

Issue #289.

tests/empty-struct.ispc

@@ -0,0 +1,13 @@
+
+export uniform int width() { return programCount; }
+
+struct Foo { };
+
+export void f_f(uniform float RET[], uniform float aFOO[]) {
+    uniform Foo f;
+    RET[programIndex] = sizeof(f); 
+}
+
+export void result(uniform float RET[]) {
+    RET[programIndex] = 1;
+}