arm64: dump: Fix implicit inclusion of definition for PCI_IOBASE

Since c9465b4e

 (arm64: add support to dump the kernel page tables)
allmodconfig has failed to build on arm64 as a result of:

../arch/arm64/mm/dump.c:55:20: error: 'PCI_IOBASE' undeclared here (not in a function)

Fix this by explicitly including io.h to ensure that a definition is
present.

Signed-off-by: Mark Brown <broonie@kernel.org>
Signed-off-by: Will Deacon <will.deacon@arm.com>