scripts: west: completion: bash: add snippet arguments to build command

Add snippet arguments to the "west build" command completion.

Signed-off-by: Henrik Brix Andersen <hebad@vestas.com>
diff --git a/scripts/west_commands/completion/west-completion.bash b/scripts/west_commands/completion/west-completion.bash
index cff8817..68684c0 100644
--- a/scripts/west_commands/completion/west-completion.bash
+++ b/scripts/west_commands/completion/west-completion.bash
@@ -702,6 +702,7 @@
 
 	local special_opts="
 		--board -b
+		--snippet -S
 		--pristine -p
 	"