build disabling alias (patch)

[Mike Jonkmans]

On devel (94bce520):
        ./configure --disable-alias; make
chokes on pcomplete.c:
        pcomplete.c:371:9: error: ‘alias_list’ undeclared (first use in this 
function); did you mean ‘alias_t’?
                371 |   free (alias_list);

Easy fix attached.

This statement has been introduced 12 years ago in devel
(10 years ago in master, bash 4.3).
Since then nobody disabled aliases or took the trouble of reporting.

Wouldn't it be opportune to remove the `--enable-alias' option?
Saves a couple of #ifdef's too.

-- 
Regards, Mike Jonkmans

patch
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