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/cursor_shape.h

@@ -1,5 +1,4 @@
-#ifndef NVIM_CURSOR_SHAPE_H
-#define NVIM_CURSOR_SHAPE_H
+#pragma once
 
 #include "nvim/api/private/defs.h"
 #include "nvim/types.h"
@@ -58,4 +57,3 @@ extern cursorentry_T shape_table[SHAPE_IDX_COUNT];
 #ifdef INCLUDE_GENERATED_DECLARATIONS
 # include "cursor_shape.h.generated.h"
 #endif
-#endif  // NVIM_CURSOR_SHAPE_H