kconfig: be more helpful if pkg-config is missing

If ncurses is installed, but at a non-default location, the previous
error message was not helpful in resolving the situation.  Now it will
suggest that pkg-config might need to be installed in addition to
ncurses.

Signed-off-by: Alyssa Ross <hi@alyssa.is>
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>