x86/boot/build: Remove always empty $(USERINCLUDE)

Commmit b6eea87f:

  ("x86, boot: Explicitly include autoconf.h for hostprogs")

correctly noted:

    [...] that because $(USERINCLUDE) isn't exported by
    the top-level Makefile it's actually empty in arch/x86/boot/Makefile.

So let's do the sane thing and remove the reference to that make variable.
Signed-off-by: Paul Bolle <pebolle@tiscali.nl>
Reviewed-by: Matt Fleming <matt@codeblueprint.co.uk>
Cc: David Howells <dhowells@redhat.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/1478166468-9760-1-git-send-email-pebolle@tiscali.nlSigned-off-by: Ingo Molnar <mingo@kernel.org>

arch/x86/boot/Makefile

@@ -65,7 +65,7 @@ clean-files += cpustr.h
 
 # ---------------------------------------------------------------------------
 
-KBUILD_CFLAGS	:= $(USERINCLUDE) $(REALMODE_CFLAGS) -D_SETUP
+KBUILD_CFLAGS	:= $(REALMODE_CFLAGS) -D_SETUP
 KBUILD_AFLAGS	:= $(KBUILD_CFLAGS) -D__ASSEMBLY__
 GCOV_PROFILE := n
 UBSAN_SANITIZE := n