UML: fix defconfig build again

Reported by Al Viro.

This fixes it:

     [AC]FLAGS -> KBUILD_[AC]FLAGS conversion in Makefile-i386.

Signed-off-by: Jeff Dike <jdike@linux.intel.com>
Cc: Al Viro <viro@ftp.linux.org.uk>
Cc: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>