web: Move declaration to avoid unused-variable warning

The variable is obviously only needed inside those `!defined(ETHERNET)`
areas.

Fixes: a80167912c ("added missing files")

src/web/web.h

@@ -183,8 +183,8 @@ class Web {
             mUploadFp.write(data, len);
             if (final) {
                 mUploadFp.close();
-                char pwd[PWD_LEN];
                 #if !defined(ETHERNET)
+                char pwd[PWD_LEN];
                 strncpy(pwd, mConfig->sys.stationPwd, PWD_LEN); // backup WiFi PWD
                 #endif
                 if (!mApp->readSettings("/tmp.json")) {