contrib: link zpool to zfs in bash-completion

Currently user won't have completion of `zpool` command until they
trigger completion of `zfs` first. This patch adds a link to `zfs`,
thus user can use both to initialize the completion.

Fixes: #16320

Signed-off-by: Shengqi Chen <harry-chen@outlook.com>

contrib/bash_completion.d/.gitignore

@@ -1 +1,2 @@
 /zfs
+/zpool

contrib/bash_completion.d/Makefile.am

@@ -1,5 +1,9 @@
 nodist_bashcompletion_DATA  = %D%/zfs
+dist_bashcompletion_DATA    = %D%/zpool
 SUBSTFILES                 += $(nodist_bashcompletion_DATA)
 
 SHELLCHECKSCRIPTS   += $(nodist_bashcompletion_DATA)
 $(call SHELLCHECK_OPTS,$(nodist_bashcompletion_DATA)): SHELLCHECK_SHELL = bash
+
+%D%/zpool: %D%/zfs
+	$(LN_S) zfs $@