[lldb] fix building with panel.h being in /usr/include/ncurses/

My openSUSE 15.2 has /usr/include/curses.h as a symlink to
/usr/include/ncurses/curses.h , but there's no such symlink
for panel.h . Prefer using /usr/include/ncurses for the includes
if they are found there by the CMake check.

Differential Revision: https://reviews.llvm.org/D85219