iommufd: Fix building without dmabuf

iommufd: Fix building without dmabuf

When DMABUF is disabled, trying to use it causes a link failure:

x86_64-linux-ld: drivers/iommu/iommufd/io_pagetable.o: in function `iopt_map_file_pages':
io_pagetable.c:(.text+0x1735): undefined reference to `dma_buf_get'
x86_64-linux-ld: io_pagetable.c:(.text+0x1775): undefined reference to `dma_buf_put'

Fixes: 44ebaa1744fd ("iommufd: Accept a DMABUF through IOMMU_IOAS_MAP_FILE")
Link: https://patch.msgid.link/r/20251204100333.1034767-1-arnd@kernel.org
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>