PCI: endpoint: Fix Kconfig dependency

PCI: endpoint: Fix Kconfig dependency

If CONFIG_NTB is not set and CONFIG_PCI_EPF_VNTB is y.

make ARCH=x86_64 CROSS_COMPILE=x86_64-linux-gnu-, will be failed, like this:

drivers/pci/endpoint/functions/pci-epf-vntb.o: In function `epf_ntb_cmd_handler':
pci-epf-vntb.c:(.text+0x95e): undefined reference to `ntb_db_event'
pci-epf-vntb.c:(.text+0xa1f): undefined reference to `ntb_link_event'
pci-epf-vntb.c:(.text+0xa42): undefined reference to `ntb_link_event'
drivers/pci/endpoint/functions/pci-epf-vntb.o: In function `pci_vntb_probe':
pci-epf-vntb.c:(.text+0x1250): undefined reference to `ntb_register_device'

The functions ntb_*() are defined in drivers/ntb/core.c, which need CONFIG_NTB setting y to be build-in.
To fix this build error, add depends on NTB.

Reported-by: Hulk Robot <hulkci@huawei.com>
Fixes: ff32fac00d97("NTB: EPF: support NTB transfer between PCI RC and EP connection")
Signed-off-by: Ren Zhijie <renzhijie2@huawei.com>
Acked-by: Frank Li <frank.li@nxp.com>
Acked-by: Randy Dunlap <rdunlap@infradead.org>
Tested-by: Randy Dunlap <rdunlap@infradead.org> # build-tested
Reported-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Jon Mason <jdmason@kudzu.us>