tests: threads::diff: fix warning for unused variable

The threads::diff test suite has a static variable `_retries`, which is
used on Windows platforms only. As it is unused on other systems, the
compiler throws a warning there. Fix the warning by wrapping the
declaration in an ifdef.

tests/threads/diff.c

@@ -19,7 +19,9 @@ static git_repository *_repo;
 static git_tree *_a, *_b;
 static git_atomic _counts[4];
 static int _check_counts;
+#ifdef GIT_WIN32
 static int _retries;
+#endif
 
 #define THREADS 20