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/globals.h

@@ -1,5 +1,4 @@
-#ifndef NVIM_GLOBALS_H
-#define NVIM_GLOBALS_H
+#pragma once
 
 #include <inttypes.h>
 #include <stdbool.h>
@@ -1103,5 +1102,3 @@ EXTERN bool skip_win_fix_cursor INIT( = false);
 EXTERN bool skip_win_fix_scroll INIT( = false);
 /// Skip update_topline() call while executing win_fix_scroll().
 EXTERN bool skip_update_topline INIT( = false);
-
-#endif  // NVIM_GLOBALS_H