ARM: Fix build regression on SA11x0, PXA, and H720x targets

Build errors similar this appeared in todays kautobuild for the above
targets:

In file included from arch/arm/include/asm/pgtable.h:461,
                 from arch/arm/mach-pxa/generic.c:26:
include/asm-generic/pgtable.h: In function 'ptep_test_and_clear_young':
include/asm-generic/pgtable.h:29: error: dereferencing pointer to incomplete type

None of the .c files including asm/pgtable.h with this error is using
this header, so simply remove the include.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>