utmp-wtmp: replace _PATH_WTMPX -> WTMPX_FILE

_PATH_WTMPX is a kind of internal definition in glibc, and
WTMPX_FILE should be an exposed definition. Both are same,
let's use WTMPX_FILE.
Note, for utmp, we use UTMPX_FILE, rather than _PATH_UTMPX.
Let's use consistent macros.

src/shared/utmp-wtmp.c

@@ -104,7 +104,7 @@ static int write_entry_wtmp(const struct utmpx *store) {
          * simply appended to the end; i.e. basically a log. */
 
         errno = 0;
-        updwtmpx(_PATH_WTMPX, store);
+        updwtmpx(WTMPX_FILE, store);
         if (errno == ENOENT) {
                 /* If utmp/wtmp have been disabled, that's a good thing, hence ignore the error. */
                 log_debug_errno(errno, "Not writing wtmp: %m");