spl: disk: usb: Add dependencies to sprintf/strto*

spl: disk: usb: Add dependencies to sprintf/strto*

If SPL serial support is disabled nothing brings in sprintf, snprintf
or simple_strtoul:

  env/built-in.o: In function `regex_callback':
  env/attr.c:128: undefined reference to `sprintf'
  disk/built-in.o: In function `blk_get_device_by_str':
  disk/part.c:386: undefined reference to `simple_strtoul'
  disk/part.c:395: undefined reference to `simple_strtoul'
  disk/built-in.o: In function `blk_get_device_part_str':
  disk/part.c:522: undefined reference to `simple_strtoul'
  disk/built-in.o: In function `part_set_generic_name':
  disk/part.c:704: undefined reference to `sprintf'
  drivers/built-in.o: In function `init_peripheral_ep':
  drivers/usb/musb-new/musb_gadget.c:1826: undefined reference to `sprintf'
  drivers/built-in.o: In function `musb_core_init':
  drivers/usb/musb-new/musb_core.c:1451: undefined reference to `snprintf'

Add those dependencies here.

Signed-off-by: Alex Kiernan <alex.kiernan@gmail.com>