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/lua/xdiff.h

@@ -1,5 +1,4 @@
-#ifndef NVIM_LUA_XDIFF_H
-#define NVIM_LUA_XDIFF_H
+#pragma once
 
 #include <lauxlib.h>
 #include <lua.h>
@@ -8,5 +7,3 @@
 #ifdef INCLUDE_GENERATED_DECLARATIONS
 # include "lua/xdiff.h.generated.h"
 #endif
-
-#endif  // NVIM_LUA_XDIFF_H