readline: correct pkg-config dependency for termcap

readline uses ncurses for terminal capabilities database, but it fails to
specify it correctly in the pkg-config .pc file, resulting in:
Requires.private: termcap

As ncurses by default provides newer terminfo instead of termcap, there's
no termcap.pc in the system and pkg-config fails when linking with readline:
readline.pc X-> termcap.pc

Help configure script to set pkg-config to use ncurses for the correct
terminal capabilities database:
Requires.private: ncurses

This fixes pkg-config dependency chain:
readline.pc -> ncurses.pc -> tinfo.pc

Signed-off-by: Denys Dmytriyenko <denis@denix.org>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>