Force the test's main function to use cdecl under Windows

Signed-off-by: Carlos Martín Nieto <carlos@cmartin.tk>

tests/test_main.c

@@ -69,7 +69,12 @@ static libgit2_suite suite_methods[]= {
 
 #define GIT_SUITE_COUNT (ARRAY_SIZE(suite_methods))
 
-int main(int GIT_UNUSED(argc), char *GIT_UNUSED(argv[]))
+#ifdef GIT_WIN32
+int __cdecl
+#else
+int
+#endif
+main(int GIT_UNUSED(argc), char *GIT_UNUSED(argv[]))
 {
 	unsigned int i, failures;