Use prototype from tsearch(3), fixes the following warning when
targetin Android arm API 17.

../../src/misc/variables.c:154:32: warning: passing 'void **' to
parameter of type 'const void **' discards qualifiers in nested
pointer types [-Wincompatible-pointer-types-discards-qualifiers]

    pp_var = tfind( &psz_name, &priv->var_root, varcmp );
                               ^~~~~~~~~~~~~~~