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>
diff --git a/disk/Kconfig b/disk/Kconfig
index 8af2a95..28fb81c 100644
--- a/disk/Kconfig
+++ b/disk/Kconfig
@@ -4,6 +4,10 @@
 config PARTITIONS
 	bool "Enable Partition Labels (disklabels) support"
 	default y
+	select SPL_SPRINTF if SPL
+	select TPL_SPRINTF if TPL
+	select SPL_STRTO if SPL
+	select TPL_STRTO if TPL
 	help
 	  Partition Labels (disklabels) Supported:
 	  Zero or more of the following: