tst_seatv4: fix compilation with GCC/C++20

Explicitly defaulting the default ctor suppresses aggregate
initialization. It's also a pointless thing to do, so don't do it.

Pick-to: 6.2 5.15
Change-Id: I8ec14f22ae1ead2dd6db643a6462f719fda344b8
Reviewed-by: David Edmundson <davidedmundson@kde.org>

tests/auto/wayland/seatv4/tst_seatv4.cpp

@@ -287,8 +287,6 @@ void tst_seatv4::simpleAxis()
         }
         struct Event // Because I didn't find a convenient way to copy it entirely
         {
-            Event() = default;
-
             QPoint pixelDelta;
             QPoint angleDelta; // eights of a degree, positive is upwards, left
         };