Disallowed 0 byte structs.

This is because they are incompatible with C++ and possibly other
languages that make them minimum size 1 (to make sure multiple
such objects don't reside at the same address). Forcing them to size
1 was also not practical, as that is requires updating the logic
of a lot of implementations and thus possibly backwards incompatible.

More here: https://github.com/google/flatbuffers/issues/4122

Change-Id: I2bfdc8597b7cfd2235bb4074bb2ae06f81f8e57d

tests/include_test/sub/include_test2.fbs

@@ -5,7 +5,7 @@ namespace MyGame.OtherNameSpace;
 
 enum FromInclude:long { IncludeVal }
 
-struct Unused {}
+struct Unused { a:int; }
 
 table TableB {
   a:TableA;