Fix AVR test failure (fixture renames main)

tests/enum_sizes/enum_intsize_unittests.cc

diff --git a/tests/enum_sizes/enum_intsize_unittests.cc b/tests/enum_sizes/enum_intsize_unittests.cc
index e89d8f3..dae8737 100644
--- a/tests/enum_sizes/enum_intsize_unittests.cc
+++ b/tests/enum_sizes/enum_intsize_unittests.cc
@@ -2,7 +2,7 @@
 #include "enum_intsize.pb.hpp"
 #include "unittests.h"
 
-int main()
+extern "C" int main()
 {
   int status = 0;