tools/mpy-tool.py: Fix build error when no qstrs present in frozen mpy.

If you happen to only have a really simple frozen file that doesn't contain
any new qstrs then the generated frozen_mpy.c file contains an empty
enumeration which causes a C compile time error.

tools/mpy-tool.py

@@ -510,6 +510,7 @@ def freeze_mpy(base_qstrs, raw_codes):
     print('#endif')
     print()
 
+    if len(new) > 0:
         print('enum {')
         for i in range(len(new)):
             if i == 0: