Makes some parameters const in libiberty's hashtab library.

include/ChangeLog:

        * hashtab.h (htab_remove_elt): Make a parameter const.
        (htab_remove_elt_with_hash): Likewise.

libiberty/ChangeLog:

        * hashtab.c (htab_remove_elt): Make a parameter const.
        (htab_remove_elt_with_hash): Likewise.