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/getchar.c

@@ -396,7 +396,7 @@ int stuff_empty(void)
  * Return TRUE if readbuf1 is empty.  There may still be redo characters in
  * redbuf2.
  */
-int readbuf1_empty()
+int readbuf1_empty(void)
 {
   return (readbuf1.bh_first.b_next == NULL);
 }