Use = for initialization

...so that an automatic loplugin:cstylecast rewrite (with a to-be-committed
enhanced loplugin) won't rewrite that to -Werror,-Wvexing-parse

  sal_Int16 nDefaultState( sal_Int16(TRISTATE_FALSE) );

Change-Id: Ic02374a34c67284dfbbb829a76fe7af9f7599411
Reviewed-on: https://gerrit.libreoffice.org/47752Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Stephan Bergmann <sbergman@redhat.com>

forms/source/component/Button.cxx

@@ -265,7 +265,7 @@ void SAL_CALL OButtonModel::setFastPropertyValue_NoBroadcast( sal_Int32 _nHandle
     {
     case PROPERTY_ID_DEFAULT_STATE:
     {
-        sal_Int16 nDefaultState( (sal_Int16)TRISTATE_FALSE );
+        sal_Int16 nDefaultState = (sal_Int16)TRISTATE_FALSE;
         OSL_VERIFY( _rValue >>= nDefaultState );
         m_eDefaultState = (ToggleState)nDefaultState;
         impl_resetNoBroadcast_nothrow();