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/buffer.c

@@ -4484,7 +4484,7 @@ void buf_delete_signs(buf_T *buf)
 /*
  * Delete all signs in all buffers.
  */
-void buf_delete_all_signs()
+void buf_delete_all_signs(void)
 {
     buf_T *buf;     /* buffer we are checking for signs */