Use attribute(unused) in PROTECT_ERRNO

clang emits warnings about unused attribute _saved_errno_, which drown
out other—potentially useful—warnings. gcc documentation is not exactly
verbose about the effects of __attribute__((unused)) on variables, but
let's assume that it works if the unit test passes.

src/shared/cgroup-util.c

@@ -1208,7 +1208,6 @@ int cg_pid_get_path_shifted(pid_t pid, char **root, char **cgroup) {
 }
 
 int cg_path_decode_unit(const char *cgroup, char **unit){
-        _cleanup_free_ char *unescaped = NULL;
         char *p, *e, *c, *s, *k;
 
         assert(cgroup);

src/shared/util.h

@@ -644,7 +644,7 @@ static inline void _reset_errno_(int *saved_errno) {
         errno = *saved_errno;
 }
 
-#define PROTECT_ERRNO _cleanup_(_reset_errno_) int _saved_errno_ = errno
+#define PROTECT_ERRNO _cleanup_(_reset_errno_) __attribute__((unused)) int _saved_errno_ = errno
 
 struct _umask_struct_ {
         mode_t mask;