Address warning `-Wstrict-prototypes` e.g. for Clang 16

[hartwork]

Previously:

# CFLAGS='-std=gnu99 -pedantic' make CC=clang
clang -std=gnu99 -pedantic -Wall -Wextra    ttyplot.c  `pkg-config --libs ncurses` -o ttyplot
ttyplot.c:51:11: warning: a function declaration without a prototype is deprecated in all versions of C [-Wstrict-prototypes]
void usage() {
          ^
           void
ttyplot.c:87:11: warning: a function declaration without a prototype is deprecated in all versions of C [-Wstrict-prototypes]
void gethw() {
          ^
           void
ttyplot.c:142:16: warning: a function declaration without a prototype is deprecated in all versions of C [-Wstrict-prototypes]
void paint_plot() {
               ^
                void
3 warnings generated.

[tenox7]

thank you