libc: kconfig: Have MINIMAL_LIBC depend on !REQUIRES_FULL_LIBC

This prevents MINIMAL_LIBC from being selected by the user (in the
menuconfig or in a configuration file) when REQUIRES_FULL_LIBC is y.
'default' on a choice only determines the default selection, not what
symbols can be selected.

It's helpful to think of Kconfig in terms of someone going into the
menuconfig and making changes.

Signed-off-by: Ulf Magnusson <Ulf.Magnusson@nordicsemi.no>