QNetworkInterface: fix warning of unused function

That causes compilation error in developer build or when '-Werror'
command line argument is forced:

kernel/qnetworkinterface_unix.cpp:142:12: error: 'int getMtu(int, ifreq*)' defined but not used [-Werror=unused-function]
 static int getMtu(int socket, struct ifreq *req)
            ^
cc1plus: all warnings being treated as errors

So, mark 'getMtu()' as unused in the appropriate branches.

Change-Id: Ib4d74845835962bb12f56baf8e13834c032a0404
Reviewed-by: Timur Pocheptsov <timur.pocheptsov@qt.io>

src/network/kernel/qnetworkinterface_unix.cpp

@@ -341,6 +341,7 @@ QT_END_INCLUDE_NAMESPACE
 
 static QList<QNetworkInterfacePrivate *> createInterfaces(ifaddrs *rawList)
 {
+    Q_UNUSED(getMtu)
     QList<QNetworkInterfacePrivate *> interfaces;
     QSet<QString> seenInterfaces;
     QVarLengthArray<int, 16> seenIndexes;   // faster than QSet<int>
@@ -564,6 +565,7 @@ static void getAddressExtraInfo(QNetworkAddressEntry *entry, struct sockaddr *sa
 
 static QList<QNetworkInterfacePrivate *> createInterfaces(ifaddrs *rawList)
 {
+    Q_UNUSED(getMtu)
     QList<QNetworkInterfacePrivate *> interfaces;
 
     // make sure there's one entry for each interface