Mk/uses/ncurses.mk: NCURSESINC is broken?

[Gleb Popov]

This PR [1] brought my attention to the following problem with ncurses.mk.

The value of NCURSESINC can't be used as a value to -I compiler flag,
because in case of ports ncurses it is set to /usr/local/include/ncurses.
At the same time, /usr/local/include/ncurses/ncurses.h header includes
other headers in this way: #include <ncurses/ncurses_dll.h> This implies
that -I/usr/local/include should be passed to the compiler, not
-I/usr/local/include/ncurses. Is it a bug, or I'm just missing something?

[1] https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=225834