Odd conversion double -> sal_Int32 -> double

...which had been introduced into that assignment in
ScDataPilotFieldObj::createDateGroup with
7dd50f9e "CWS-TOOLING: integrate CWS dr67", but
for no documented purpose.  So lets assume the truncation is for a reason, and
replace it with a call to std::trunc.

Change-Id: I2dfd4c0f5966e9ab294ecd5b3424fece2ea70daa
Reviewed-on: https://gerrit.libreoffice.org/47995Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Kohei Yoshida <libreoffice@kohei.us>

sc/source/ui/unoobj/dapiuno.cxx

@@ -18,6 +18,8 @@
  */
 
 #include <algorithm>
+#include <cmath>
+
 #include <svl/hint.hxx>
 #include <vcl/svapp.hxx>
 
@@ -2704,7 +2706,7 @@ Reference < XDataPilotField > SAL_CALL ScDataPilotFieldObj::createDateGroup( con
         aInfo.mbAutoEnd = rInfo.HasAutoEnd;
         aInfo.mfStart = rInfo.Start;
         aInfo.mfEnd = rInfo.End;
-        aInfo.mfStep = static_cast< sal_Int32 >( rInfo.Step );
+        aInfo.mfStep = std::trunc( rInfo.Step );
 
         // create a local copy of the entire save data (will be written back below)
         ScDPSaveData aSaveData = *pDPObj->GetSaveData();