r.rgb: at least one of rgb is required (avoid r.mapcalc parse error later)

git-svn-id: https://svn.osgeo.org/grass/grass/trunk@68072 15284696-431f-4ddb-bdfa-cd5b030d7da7

scripts/r.rgb/r.rgb.py

@@ -35,6 +35,9 @@
 #% description: Blue channel raster map name
 #% required: no
 #%end
+#%rules
+#% required: red, green, blue
+#%end
 
 import grass.script as gscript