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/private/converter.h

@@ -1,5 +1,4 @@
-#ifndef NVIM_API_PRIVATE_CONVERTER_H
-#define NVIM_API_PRIVATE_CONVERTER_H
+#pragma once
 
 #include "nvim/api/private/defs.h"
 #include "nvim/eval/typval_defs.h"
@@ -7,5 +6,3 @@
 #ifdef INCLUDE_GENERATED_DECLARATIONS
 # include "api/private/converter.h.generated.h"
 #endif
-
-#endif  // NVIM_API_PRIVATE_CONVERTER_H

src/nvim/api/private/defs.h

@@ -1,5 +1,4 @@
-#ifndef NVIM_API_PRIVATE_DEFS_H
-#define NVIM_API_PRIVATE_DEFS_H
+#pragma once
 
 #include <stdbool.h>
 #include <stdint.h>
@@ -139,5 +138,3 @@ typedef struct {
 } KeySetLink;
 
 typedef KeySetLink *(*FieldHashfn)(const char *str, size_t len);
-
-#endif  // NVIM_API_PRIVATE_DEFS_H

src/nvim/api/private/dispatch.h

@@ -1,5 +1,4 @@
-#ifndef NVIM_API_PRIVATE_DISPATCH_H
-#define NVIM_API_PRIVATE_DISPATCH_H
+#pragma once
 
 #include <stdbool.h>
 #include <stdint.h>
@@ -30,5 +29,3 @@ extern const MsgpackRpcRequestHandler method_handlers[];
 # include "api/private/dispatch_wrappers.h.generated.h"
 # include "keysets_defs.generated.h"
 #endif
-
-#endif  // NVIM_API_PRIVATE_DISPATCH_H

src/nvim/api/private/helpers.h

@@ -1,5 +1,4 @@
-#ifndef NVIM_API_PRIVATE_HELPERS_H
-#define NVIM_API_PRIVATE_HELPERS_H
+#pragma once
 
 #include <stdbool.h>
 #include <stddef.h>
@@ -199,5 +198,3 @@ typedef struct {
     current_channel_id = save_channel_id; \
     current_sctx = save_current_sctx; \
   } while (0);
-
-#endif  // NVIM_API_PRIVATE_HELPERS_H

src/nvim/api/private/validate.h

@@ -1,5 +1,4 @@
-#ifndef NVIM_API_PRIVATE_VALIDATE_H
-#define NVIM_API_PRIVATE_VALIDATE_H
+#pragma once
 
 #include <stdbool.h>
 #include <stddef.h>
@@ -95,5 +94,3 @@
 #ifdef INCLUDE_GENERATED_DECLARATIONS
 # include "api/private/validate.h.generated.h"
 #endif
-
-#endif  // NVIM_API_PRIVATE_VALIDATE_H