parisc: fix build when ARCH_HAS_KMAP

When we build for PA8X00, we define ARCH_HAS_KMAP, which results in
the kmap_types.h include in highmem.h getting skipped...

In file included from include/linux/pagemap.h:10,
                 from include/linux/mempolicy.h:62,
                 from init/main.c:52:
include/linux/highmem.h:196: warning: 'enum km_type' declared inside parameter list
include/linux/highmem.h:196: warning: its scope is only this definition or declaration, which is probably not what you want
include/linux/highmem.h:196: error: parameter 1 ('type') has incomplete type

Signed-off-by: Kyle McMartin <kyle@mcmartin.ca>