Forward declare LabImage

rtengine/labimage.h

@@ -16,8 +16,9 @@
  *  You should have received a copy of the GNU General Public License
  *  along with RawTherapee.  If not, see <https://www.gnu.org/licenses/>.
  */
-#ifndef _LABIMAGE_H_
-#define _LABIMAGE_H_
+#pragma once
+
+#include <cstring>
 
 namespace rtengine
 {
@@ -45,4 +46,3 @@ public:
 };
 
 }
-#endif