ncpfs: fix build warning of strncpy

Not upstream as ncpfs is long deleted.

Fix up two strncpy build warnings in ncp_get_charsets() by using strscpy
and the max size of the array.

It's not like anyone uses this code anyway, and this gets rid of two
build warnings so that we can see real warnings as they pop up over
time.

Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>