qwaitcondition_p.h: fix -Wunused-const-variable

Make the constexpr variable inline instead of static to shut up the
warning.

Amends fbc491230f.

Pick-to: 6.7
Task-number: QTBUG-126219
Change-Id: Ifa8db14dc28f039d0dc65eb779d715cd4a2c7240
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
(cherry picked from commit 743403cb6d9957c1bdb6ef7a1bb6f30377f578a5)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

src/corelib/thread/qwaitcondition_p.h

@@ -28,7 +28,7 @@ namespace QtPrivate {
 // Ideal alignment for mutex and condition_variable: it's the hardware
 // interference size (size of a cache line) if the types are likely to contain
 // the actual data structures, otherwise just that of a pointer.
-static constexpr quintptr IdealMutexAlignment =
+inline constexpr quintptr IdealMutexAlignment =
         sizeof(std::mutex) > sizeof(void *) &&
         sizeof(std::condition_variable) > sizeof(void *) ?
             64 : alignof(void*);