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/eval/executor.h

@@ -1,5 +1,4 @@
-#ifndef NVIM_EVAL_EXECUTOR_H
-#define NVIM_EVAL_EXECUTOR_H
+#pragma once
 
 #include "nvim/eval/typval_defs.h"
 
@@ -8,4 +7,3 @@ extern char *e_list_index_out_of_range_nr;
 #ifdef INCLUDE_GENERATED_DECLARATIONS
 # include "eval/executor.h.generated.h"
 #endif
-#endif  // NVIM_EVAL_EXECUTOR_H