Fix ncurses detection at configure time

There appears to be a typo/thinko in the ncurses detection
stuff in configure.ac.  The fallout was that on a Linux host
without ncurses-devel installed, "./configure" failed to find
curses.h but claimed overall success, then the build failed
later during make.  This patch removes an extraneous comma, which
seems to make it both fail and succeed correctly when it should.