iommufd/selftest: Add missing kconfig for DMA_SHARED_BUFFER

The test doesn't build without it, dma-buf.h does not provide stub
functions if it is not enabled. Compilation can fail with:

 ERROR:root:ld: vmlinux.o: in function `iommufd_test':
 (.text+0x3b1cdd): undefined reference to `dma_buf_get'
 ld: (.text+0x3b1d08): undefined reference to `dma_buf_put'
 ld: (.text+0x3b2105): undefined reference to `dma_buf_export'
 ld: (.text+0x3b211f): undefined reference to `dma_buf_fd'
 ld: (.text+0x3b2e47): undefined reference to `dma_buf_move_notify'

Add the missing select.

Fixes: d2041f1f ("iommufd/selftest: Add some tests for the dmabuf flow")
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>
Signed-off-by: Joerg Roedel <joerg.roedel@amd.com>