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/os/env.c

@@ -55,7 +55,7 @@ char *os_getenvname_at_index(size_t index)
 /// Get the process ID of the Neovim process.
 ///
 /// @return the process ID.
-int64_t os_get_pid()
+int64_t os_get_pid(void)
 {
 #ifdef _WIN32
   return (int64_t)GetCurrentProcessId();