Enforce disallowing empty structs

This patch enfoces an error when it encounters an empty struct,
effectively eliminating the deprecation warning.

Also adjust 419_interface_structs to explicitely test for (non-empty) structs,
as this behaviour "may" change in the future.

test/libsolidity/SolidityEndToEndTest.cpp

@@ -1271,7 +1271,6 @@ BOOST_AUTO_TEST_CASE(deleteStruct)
 		contract test {
 			struct topStruct {
 				nestedStruct nstr;
-				emptyStruct empty;
 				uint topValue;
 				mapping (uint => uint) topMapping;
 			}
@@ -1281,8 +1280,6 @@ BOOST_AUTO_TEST_CASE(deleteStruct)
 				uint nestedValue;
 				mapping (uint => bool) nestedMapping;
 			}
-			struct emptyStruct{
-			}
 			function test(){
 				toDelete = 5;
 				str.topValue = 1;