Updating bash completion build file

Commit 46ebd0a updated the build system to make symbolic link for zpool.
However, this commit did not update the automake file to also add the
symbolic link to the CLEANFILES variable. This is necessary so the link
is removed when running make clean/distclean.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Tino Reichardt <milky-zfs@mcmilk.de>
Signed-off-by: Brian Atkinson <batkinson@lanl.gov>
Closes openzfs#16422

contrib/bash_completion.d/Makefile.am

@@ -1,6 +1,7 @@
 nodist_bashcompletion_DATA  = %D%/zfs %D%/zpool
 COMPLETION_FILES            = %D%/zfs
-SUBSTFILES                 += $(COMPLETION_FILES)
+SUBSTFILES                  += $(COMPLETION_FILES)
+CLEANFILES                  += %D%/zpool
 
 SHELLCHECKSCRIPTS   += $(COMPLETION_FILES)
 $(call SHELLCHECK_OPTS,$(COMPLETION_FILES)): SHELLCHECK_SHELL = bash