Fix warnings in stereo/digital_elevation_map.h

> digital_elevation_map.h:39:9: warning: unknown pragma ignored [-Wunknown-pragmas]

`#pragma warning(disable : 4996)` is MSVS-only. Further, it seems to be
added here to prevent warnings originating from "point_types.h".
Therefore if we really need it, it should be re-added with MSVC ifdef
and directly in "point_types.h".

stereo/include/pcl/stereo/digital_elevation_map.h

@@ -36,10 +36,8 @@
 #ifndef PCL_DIGITAL_ELEVATION_MAP_H_
 #define PCL_DIGITAL_ELEVATION_MAP_H_
 
-#pragma warning(disable : 4996)
 #include <pcl/point_types.h>
 #include <pcl/stereo/disparity_map_converter.h>
-#pragma warning(default : 4996)
 
 namespace pcl
 {