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.
diff --git a/tests/threads/diff.c b/tests/threads/diff.c
index 90d35b1..2560402 100644
--- a/tests/threads/diff.c
+++ b/tests/threads/diff.c
@@ -19,7 +19,9 @@
static git_tree *_a, *_b;
static git_atomic _counts[4];
static int _check_counts;
+#ifdef GIT_WIN32
static int _retries;
+#endif
#define THREADS 20
