basic/glob-util: do not use names with "_" prefix

Names starting with _ or __ are reserved by the standard, better to
avoid them.

src/test/test-glob-util.c

@@ -49,7 +49,7 @@ static void test_glob_exists(void) {
         assert_se(r == 0);
 }
 
-static void _closedir(void* v) {
+static void closedir_wrapper(void* v) {
         (void) closedir(v);
 }
 
@@ -58,7 +58,7 @@ static void test_glob_no_dot(void) {
         const char *fn;
 
         _cleanup_globfree_ glob_t g = {
-                .gl_closedir = _closedir,
+                .gl_closedir = closedir_wrapper,
                 .gl_readdir = (struct dirent *(*)(void *)) readdir_no_dot,
                 .gl_opendir = (void *(*)(const char *)) opendir,
                 .gl_lstat = lstat,