cfdisk: fix missing prototype for `get_wch`

cfdisk: fix missing prototype for `get_wch`

The header <ncursesw/ncurses.h> defines the get_wch(3) function only
when `NCURSES_WIDECHAR` is defined. This define is actually getting set
in the same header file, but only in case `_XOPEN_SOURCE` is defined and
has a value of 500 or higher. As we already have the precedence of
defining `_XOPEN_SOURCE` to a value of 600 in some other files, simply
define it to the minimum required value of 500 in "cfdisk.c". This
silences a warning for `get_wch` being unknown.

Signed-off-by: Patrick Steinhardt <ps@pks.im>