vfio/pci: zap_vma_ptes() needs MMU

zap_vma_ptes() is only available when CONFIG_MMU is set/enabled.
Without CONFIG_MMU, vfio_pci.o has build errors, so make
VFIO_PCI depend on MMU.

riscv64-linux-ld: drivers/vfio/pci/vfio_pci.o: in function `vfio_pci_mmap_open':
vfio_pci.c:(.text+0x1ec): undefined reference to `zap_vma_ptes'
riscv64-linux-ld: drivers/vfio/pci/vfio_pci.o: in function `.L0 ':
vfio_pci.c:(.text+0x165c): undefined reference to `zap_vma_ptes'

Fixes: 11c4cd07

 ("vfio-pci: Fault mmaps to enable vma tracking")
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Reported-by: kernel test robot <lkp@intel.com>
Cc: Alex Williamson <alex.williamson@redhat.com>
Cc: Cornelia Huck <cohuck@redhat.com>
Cc: kvm@vger.kernel.org
Cc: Jason Gunthorpe <jgg@nvidia.com>
Cc: Eric Auger <eric.auger@redhat.com>
Message-Id: <20210515190856.2130-1-rdunlap@infradead.org>
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>