test_setattr: fix const cppcheck warning

Add const to 'mountpoint' function parameter.

Fixes cppcheck warning:
- cppcheck-constParameterPointer

Signed-off-by: Bernd Schubert <bernd@bsbernd.com>

test/test_setattr.c

@@ -134,7 +134,8 @@ static void* run_fs(void *data) {
     return NULL;
 }
 
-static void test_fs(char *mountpoint) {
+static void test_fs(const char *mountpoint)
+{
     char fname[PATH_MAX];
     int fd;