bash-completion: umount explicitly needs gawk

gensub() is a gawk extension, so use it explicitly. Otherwise users will
get an error if their 'awk' defaults to something else.

Fixes: 3ebfc8d37 ("bash-completion: umount support relative path and ~ as home shorthands")
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>

bash-completion/umount

@@ -51,7 +51,7 @@ _umount_module()
 
 	local oldifs=$IFS
 	IFS=$'\n'
-	COMPREPLY=( $( compgen -W "$(findmnt -lno TARGET | awk \
+	COMPREPLY=( $( compgen -W "$(findmnt -lno TARGET | gawk \
 		'{
 			if ($0 ~ ENVIRON["HOME"]) {
 				homeless = $0