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>

arch/um/Makefile-i386

@@ -22,8 +22,8 @@ export LDFLAGS HOSTCFLAGS HOSTLDFLAGS UML_OBJCOPYFLAGS
 endif
 endif
 
-CFLAGS			+= -DCONFIG_X86_32
-AFLAGS			+= -DCONFIG_X86_32
+KBUILD_CFLAGS		+= -DCONFIG_X86_32
+KBUILD_AFLAGS		+= -DCONFIG_X86_32
 CONFIG_X86_32		:= y
 export CONFIG_X86_32