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.

scripts/update_terminfo.sh

@@ -63,12 +63,9 @@ sorted_terms="$(echo "${!entries[@]}" | tr ' ' '\n' | sort | xargs)"
 cat > "$target" <<EOF
 // uncrustify:off
 
-//
 // Generated by scripts/update_terminfo.sh and $(tic -V)
-//
 
-#ifndef NVIM_TUI_TERMINFO_DEFS_H
-#define NVIM_TUI_TERMINFO_DEFS_H
+#pragma once
 
 #include <stdint.h>
 EOF
@@ -87,7 +84,4 @@ for term in $sorted_terms; do
   printf '};\n'
 done >> "$target"
 
-cat >> "$target" <<EOF
-#endif  // NVIM_TUI_TERMINFO_DEFS_H
-EOF
 print_bold 'done\n'