Fix compiler warning from discarded return value

Amend 53b7cb1bd7, match() is only used
to provoke data races.

Change-Id: Id20b11fedf7f20e74baab15bbb60c995c1a0c794
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

tests/auto/corelib/text/qregularexpression/tst_qregularexpression.cpp

@@ -2132,7 +2132,7 @@ private:
     {
         yieldCurrentThread();
         for (int i = 0; i < MATCH_ITERATIONS; ++i)
-            m_re.match(m_subject);
+            (void)m_re.match(m_subject);
     }
 
     const QRegularExpression &m_re;