Makefile.inc1: Add a missing ${dist} to distributeworld for usr/include/i386

Not including ${dist} results in the following non-fatal error printed
once per extra distribution:

mkdir  //usr/obj/usr/src/amd64.amd64/release/dist/usr/include/i386
mkdir: //usr/obj/usr/src/amd64.amd64/release/dist/usr/include: No such file or directory
*** Error code 1 (ignored)

Also fix a whitespace nit on this line whilst here.

Reviewed by: brooks
Fixes: a09ea2bbc305 ("amd64: add an i386 include directory")