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/api/private/handle.c

@@ -33,7 +33,7 @@ HANDLE_IMPL(buf_T, buffer)
 HANDLE_IMPL(win_T, window)
 HANDLE_IMPL(tabpage_T, tabpage)
 
-void handle_init()
+void handle_init(void)
 {
   HANDLE_INIT(buffer);
   HANDLE_INIT(window);

src/nvim/api/private/helpers.c

@@ -22,7 +22,7 @@
 #endif
 
 /// Start block that may cause vimscript exceptions
-void try_start()
+void try_start(void)
 {
   ++trylevel;
 }