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/api/extmark.h

@@ -1,5 +1,4 @@
-#ifndef NVIM_API_EXTMARK_H
-#define NVIM_API_EXTMARK_H
+#pragma once
 
 #include "nvim/api/keysets.h"
 #include "nvim/api/private/defs.h"
@@ -14,4 +13,3 @@ EXTERN handle_T next_namespace_id INIT( = 1);
 #ifdef INCLUDE_GENERATED_DECLARATIONS
 # include "api/extmark.h.generated.h"
 #endif
-#endif  // NVIM_API_EXTMARK_H