make Ubuntu's gcc happy

git-svn-id: https://svn.osgeo.org/grass/grass/trunk@53072 15284696-431f-4ddb-bdfa-cd5b030d7da7

raster/r.viewshed/main.cpp

@@ -44,6 +44,7 @@
 extern "C"
 {
 #include <grass/config.h>
+#include <grass/raster.h>
 #include <grass/gis.h>
 #include <grass/glocale.h>
 }