Suppress compiler warnings in dshash.c.

Suppress compiler warnings in dshash.c.

Some compilers complain, not unreasonably, about left-shifting an
int32 "1" and then assigning the result to an int64.  In practice
I sure hope that this data structure never gets large enough that
an overflow would actually occur; but let's cast the constant to
the right type to avoid the hazard.

In passing, fix a typo in dshash.h.

Amit Kapila, adjusted as per comment from Thomas Munro.

Discussion: https://postgr.es/m/CAA4eK1+5vfVMYtjK_NX8O3-42yM3o80qdqWnQzGquPrbq6mb+A@mail.gmail.com