irqtop: add vw_printw() fallback for slang builds

irqtop: add vw_printw() fallback for slang builds

The vw_printw() function and OK macro are ncurses-specific and not
available in slang's curses compatibility layer. This causes build
failures when configuring with --with-slang.

Add configure/meson checks for vw_printw() availability and provide
a portable fallback implementation using vsnprintf() and waddstr()
when the function is not available.

Addresses: https://github.com/util-linux/util-linux/issues/4177
Signed-off-by: Karel Zak <kzak@redhat.com>