Use strict function prototypes #945

`-Wstrict-prototypes` warn if a function is declared or defined without
specifying the argument types.

This warning disallow function prototypes with empty parameter list.
In C, a function declared with an empty parameter list accepts an
arbitrary number of arguments when being called. This is for historic
reasons; originally, C functions didn't have prototypes, as C evolved
from B, a typeless language. When prototypes were added, the original
typeless declarations were left in the language for backwards
compatibility.
Instead we should provide `void` in argument list to state
that function doesn't have arguments.

Also this warning disallow declaring type of the parameters after the
parentheses because Neovim header generator produce no declarations for
old-stlyle prototypes: it expects to find `{` after prototype.

src/nvim/screen.c

@@ -639,7 +639,7 @@ void update_single_line(win_T *wp, linenr_T lnum)
  * Prepare for updating one or more windows.
  * Caller must check for "updating_screen" already set to avoid recursiveness.
  */
-static void update_prepare()
+static void update_prepare(void)
 {
     cursor_off();
     updating_screen = TRUE;
@@ -649,7 +649,7 @@ static void update_prepare()
 /*
  * Finish updating one or more windows.
  */
-static void update_finish()
+static void update_finish(void)
 {
     if (redraw_cmdline) {
         showmode();