Silence warning about unused parameter

Happened on an iOS build

io/qlockfile_unix.cpp:217:51: error: unused parameter 'pid' [-Werror,-
Wunused-parameter]

Change-Id: Id3d5c7bf4d4c45069621ffff13f7f7a30728c071
Reviewed-by: Jake Petroules <jake.petroules@petroules.com>

src/corelib/io/qlockfile_unix.cpp

@@ -241,6 +241,7 @@ QString QLockFilePrivate::processNameByPid(qint64 pid)
     free(proc);
     return name;
 #else
+    Q_UNUSED(pid);
     return QString();
 #endif
 }