QSysInfo: fix warning on non-Linux about unconditional adjacent returns

E.g. as seen on Integrity:

    "global/qglobal.cpp", line 2967: warning #111-D: statement is unreachable

Change-Id: I6845192ee7fb14b66700b68118355c871b1a4baf
Reviewed-by: Mårten Nordheim <marten.nordheim@qt.io>

src/corelib/global/qglobal.cpp

@@ -2952,6 +2952,7 @@ QString QSysInfo::machineHostName()
     struct utsname u;
     if (uname(&u) == 0)
         return QString::fromLocal8Bit(u.nodename);
+    return QString();
 #else
 #  ifdef Q_OS_WIN
     // Important: QtNetwork depends on machineHostName() initializing ws2_32.dll
@@ -2964,7 +2965,6 @@ QString QSysInfo::machineHostName()
     hostName[sizeof(hostName) - 1] = '\0';
     return QString::fromLocal8Bit(hostName);
 #endif
-    return QString();
 }
 #endif // QT_BOOTSTRAPPED