i386: Resolve dependency of asm-i386/pgtable.h on highmem.h

pgtable.h does not include highmem.h but uses various constants from
highmem.h.  We cannot include highmem.h because highmem.h will in turn include
many other include files that also depend on pgtable.h

So move the definitions from highmem.h into pgtable.h.

[akpm@linux-foundation.org: coding-style fixes]
Signed-off-by: Christoph Lameter <clameter@sgi.com>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>