Fix another coverity issue

rtengine/simpleprocess.cc

@@ -1260,8 +1260,8 @@ private:
 
         Imagefloat* readyImg = nullptr;
         cmsHPROFILE jprof = nullptr;
-        bool customGamma = false;
-        bool useLCMS = false;
+        constexpr bool customGamma = false;
+        constexpr bool useLCMS = false;
         bool bwonly = params.blackwhite.enabled && !params.colorToning.enabled && !autili && !butili ;
 
         ///////////// Custom output gamma has been removed, the user now has to create