clar: mark `cl_git_thread_check()` as inline

The function `cl_git_thread_check()` is defined as static. As the
function is defined in a header file which is included by our
tests, this can result in warnings for every test file where
`cl_git_thread_check` is never used.

Fix the issue by marking it as inline instead.

tests/clar_libgit2.h

@@ -79,7 +79,7 @@ typedef struct {
 	} \
 	} while (0)
 
-static void cl_git_thread_check(void *data)
+GIT_INLINE(void) cl_git_thread_check(void *data)
 {
 	cl_git_thread_err *threaderr = (cl_git_thread_err *)data;
 	if (threaderr->error != 0)