Fix warning about unused variable in QtPlatformSupport

GCC was complaining:
qevdevtouch.cpp:475:13: error: 'maxId' may be used uninitialized in this function [-Werror=maybe-uninitialized]

Which got me scratching my head: maxId was unconditionally initialised. How
could GCC be complaining about it being uninitialised? Well, turns out that
bestId could be uninitialised and the code does:

            if (bestId > maxId)
                maxId = bestId;

Of course, if bestId was uninitialised, the warning should have been in the
"if" line first.

Change-Id: I5e174ab2957d76ad040c14fa6ef8535129b6dce3
Reviewed-by: Laszlo Agocs <lagocs83@gmail.com>

src/platformsupport/input/evdevtouch/qevdevtouch.cpp

@@ -457,7 +457,7 @@ void QEvdevTouchScreenData::assignIds()
     int maxId = -1;
     QHash<int, Contact>::iterator it, ite, bestMatch;
     while (!pending.isEmpty() && !candidates.isEmpty()) {
-        int bestDist = -1, bestId;
+        int bestDist = -1, bestId = 0;
         for (it = pending.begin(), ite = pending.end(); it != ite; ++it) {
             int dist;
             int id = findClosestContact(candidates, it->x, it->y, &dist);