Add explicit casts in four places to simplehash.h

Hi hackers,

While working on an extension, I found that simplehash.h is missing
explicit casts in four places. Without these casts, compiling code
including simplehash.h yields warnings if the code is compiled with
-Wc++-compat.

PostgreSQL seems to mostly prefer omitting the explicit casts, however
there are many places where an explicit cast is actually used. Among
many others, see e.g.

bool.c:
    state = (BoolAggState *) MemoryContextAlloc(agg_context,
sizeof(BoolAggState));

domains.c:
   my_extra = (DomainIOData *) MemoryContextAlloc(mcxt,
sizeof(DomainIOData));

What about, while not being strictly necessary for PostgreSQL itself,
also adding such casts to simplehash.h so that it can be used in code
where -Wc++-compat is enabled?

Attached is a small patch that adds the aforementioned casts.
Thanks for your consideration!

--
David Geier
(ServiceNow)