readline/tcap.h: Update definitions for C23

readline/tcap.h: Update definitions for C23

C23 changes how function definitions like int `int tputs ()` are
interpreted. In older standards this meant that the function arguments
are unknown. In C23 this is interpreted as `int tputs (void)` so now
when we compile with GCC15 (which defaults to -std=gnu23) we get an
error such as

  readline/display.c:2839:17: error: too many arguments to function 'tputs'; expected 0, have 3

Add the function arguments for tgetent(), tgetflag(), tgetnum(),
tgetstr(), tputs() and tgoto().

Signed-off-by: Chris Packham <judge.packham@gmail.com>
Approved-By: Tom Tromey <tom@tromey.com>