tests: Correct warning in sw_tree1.c

At least some clang versions (correctly) warn that 'size' is used
unititialized, if sw_tree1 is invoked with argc > 2.  This corrects the
warning.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>

tests/sw_tree1.c

@@ -111,6 +111,8 @@ int main(int argc, char *argv[])
 				CONFIG("Bad allocation mode \"%s\" specified",
 				       argv[1]);
 		}
+	} else {
+		CONFIG("sw_tree1 <dtb file> [<allocation mode>]");
 	}
 
 	fdt = xmalloc(size);