BUG/MINOR: args: add missing entry for ARGT_MAP in arg_type_names

This type is currently not used in an argument so it's harmles. But
better correctly fill the name.
(cherry picked from commit 53c250e1656dda7c9fcece5b688f0f3a9a3ec068)
diff --git a/src/arg.c b/src/arg.c
index edaeee7..f23ee02 100644
--- a/src/arg.c
+++ b/src/arg.c
@@ -33,6 +33,7 @@
 	[ARGT_TAB]  = "table",
 	[ARGT_SRV]  = "server",
 	[ARGT_USR]  = "user list",
+	[ARGT_MAP]  = "map",
 	/* Unassigned types must never happen. Better crash during parsing if they do. */
 };