refactor: replace manual header guards with #pragma once

It is less error-prone than manually defining header guards. Pretty much
all compilers support it even if it's not part of the C standard.

src/nvim/input.h

@@ -1,9 +1,7 @@
-#ifndef NVIM_INPUT_H
-#define NVIM_INPUT_H
+#pragma once
 
 #include "nvim/vim.h"
 
 #ifdef INCLUDE_GENERATED_DECLARATIONS
 # include "input.h.generated.h"
 #endif
-#endif  // NVIM_INPUT_H