busybox: enable fdisk in default config

As noted by mnemoc on IRC the default busybox config doesn't enable
fdisk and util-linux is hidden by !BR2_PACKAGE_BUSYBOX_SHOW_OTHERS.
On modern systems fdisk usage is becoming more common so enable it, it
just adds up ~25 KiB to total binary size.
People who are on the edge for space savings will normally customize
their busybox config.

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>