net/9p/usbg: Fix build error

net/9p/usbg: Fix build error

When CONFIG_NET_9P_USBG=y but CONFIG_USB_LIBCOMPOSITE=m and
CONFIG_CONFIGFS_FS=m, the following build error occurs:

	riscv64-unknown-linux-gnu-ld: net/9p/trans_usbg.o: in function `usb9pfs_free_func':
	trans_usbg.c:(.text+0x124): undefined reference to `usb_free_all_descriptors'
	riscv64-unknown-linux-gnu-ld: net/9p/trans_usbg.o: in function `usb9pfs_rx_complete':
	trans_usbg.c:(.text+0x2d8): undefined reference to `usb_interface_id'
	riscv64-unknown-linux-gnu-ld: trans_usbg.c:(.text+0x2f6): undefined reference to `usb_string_id'
	riscv64-unknown-linux-gnu-ld: net/9p/trans_usbg.o: in function `usb9pfs_func_bind':
	trans_usbg.c:(.text+0x31c): undefined reference to `usb_ep_autoconfig'
	riscv64-unknown-linux-gnu-ld: trans_usbg.c:(.text+0x336): undefined reference to `usb_ep_autoconfig'
	riscv64-unknown-linux-gnu-ld: trans_usbg.c:(.text+0x378): undefined reference to `usb_assign_descriptors'
	riscv64-unknown-linux-gnu-ld: net/9p/trans_usbg.o: in function `f_usb9pfs_opts_buflen_store':
	trans_usbg.c:(.text+0x49e): undefined reference to `usb_put_function_instance'
	riscv64-unknown-linux-gnu-ld: net/9p/trans_usbg.o: in function `usb9pfs_alloc_instance':
	trans_usbg.c:(.text+0x5fe): undefined reference to `config_group_init_type_name'
	riscv64-unknown-linux-gnu-ld: net/9p/trans_usbg.o: in function `usb9pfs_alloc':
	trans_usbg.c:(.text+0x7aa): undefined reference to `config_ep_by_speed'
	riscv64-unknown-linux-gnu-ld: trans_usbg.c:(.text+0x7ea): undefined reference to `config_ep_by_speed'
	riscv64-unknown-linux-gnu-ld: net/9p/trans_usbg.o: in function `usb9pfs_set_alt':
	trans_usbg.c:(.text+0x828): undefined reference to `alloc_ep_req'
	riscv64-unknown-linux-gnu-ld: net/9p/trans_usbg.o: in function `usb9pfs_modexit':
	trans_usbg.c:(.exit.text+0x10): undefined reference to `usb_function_unregister'
	riscv64-unknown-linux-gnu-ld: net/9p/trans_usbg.o: in function `usb9pfs_modinit':
	trans_usbg.c:(.init.text+0x1e): undefined reference to `usb_function_register'

Select the config for NET_9P_USBG to fix it.

Fixes: a3be076dc174 ("net/9p/usbg: Add new usb gadget function transport")
Signed-off-by: Jinjie Ruan <ruanjinjie@huawei.com>
Tested-by: Kexy Biscuit <kexybiscuit@aosc.io>
Link: https://lore.kernel.org/r/20240930081520.2371424-1-ruanjinjie@huawei.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>