libdecnumber: Don't error out on decNumberLocal.h re-inclusion

decNumberLocal.h errors out when it's included with its header guard
defined.  This catches multiple inclusions.

Drop that.  Including it 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>
