r.volume: add missing header

git-svn-id: https://svn.osgeo.org/grass/grass/trunk@57442 15284696-431f-4ddb-bdfa-cd5b030d7da7

raster/r.volume/local_proto.h

@@ -0,0 +1,3 @@
+#define SEP "-----------------------------------------------------------------------------"
+
+int centroids(int, int *, int *, int, int);