libdecnumber: Don't fool around with guards to avoid #include

Some libdecnumber headers avoid including decNumber.h or decContext.h
again by checking their header guards.  Don't.  Including them
multiple times is safe, and the compiler can do it efficiently.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Richard Henderson <rth@twiddle.net>
