Fix build in INTEGRITY: __mulh is not defined

Commit 6b875f0 created a macro for it,
but in the multiple updates to find the best solution, we forgot to use
it.

Fixes: QTBUG-72429
Change-Id: I4ac1156702324f0fb814fffd156f27c1789d1409
Reviewed-by: Thomas Miller <[email protected]>
Reviewed-by: Janne Koskinen <[email protected]>
Reviewed-by: Mårten Nordheim <[email protected]>

src/corelib/global/qnumeric_p.h

@@ -348,7 +348,7 @@ template <> inline bool mul_overflow(qint64 v1, qint64 v2, qint64 *r)
     // as signed for the low bits and use a signed right shift to verify that
     // 'high' is nothing but sign bits that match the sign of 'low'.
 
-    qint64 high = __mulh(v1, v2);
+    qint64 high = Q_SMULH(v1, v2);
     *r = qint64(quint64(v1) * quint64(v2));
     return (*r >> 63) != high;
 }