make StopWatch more robust

rtengine/mytime.h

@@ -16,8 +16,7 @@
  *  You should have received a copy of the GNU General Public License
  *  along with RawTherapee.  If not, see <http://www.gnu.org/licenses/>.
  */
-#ifndef _MYTIME_
-#define _MYTIME_
+#pragma once
 
 #ifdef WIN32
 #include <windows.h>
@@ -58,11 +57,11 @@ public:
         t.tv_sec = tv.tv_sec;
         t.tv_nsec = tv.tv_usec * 1000;
 #else
-        clock_gettime (CLOCK_REALTIME, &t);
+        clock_gettime(CLOCK_REALTIME, &t);
 #endif
     }
 
-    int etime (MyTime a)
+    int etime (const MyTime &a) const
     {
 #ifndef WIN32
         return (t.tv_sec - a.t.tv_sec) * 1000000 + (t.tv_nsec - a.t.tv_nsec) / 1000;
@@ -71,6 +70,3 @@ public:
 #endif
     }
 };
-
-
-#endif