refactor: Replace include guards by '#pragma once' (#6322)

This change replaces all include guards in the `src/` and `include/` directories by `#pragma once`.

.github/scripts/rename/include.sh

@@ -0,0 +1,30 @@
+#!/bin/bash
+
+# Exit the script as soon as an error occurs.
+set -e
+
+# This script checks whether there are no new include guards introduced by a new
+# PR, as header files should use "#pragma once" instead. The script assumes any
+# include guards will use "XRPL_" as prefix.
+# Usage: .github/scripts/rename/include.sh <repository directory>
+
+if [ "$#" -ne 1 ]; then
+    echo "Usage: $0 <repository directory>"
+    exit 1
+fi
+
+DIRECTORY=$1
+echo "Processing directory: ${DIRECTORY}"
+if [ ! -d "${DIRECTORY}" ]; then
+    echo "Error: Directory '${DIRECTORY}' does not exist."
+    exit 1
+fi
+
+find "${DIRECTORY}" -type f \( -name "*.h" -o -name "*.hpp" -o -name "*.ipp" \) | while read -r FILE; do
+    echo "Processing file: ${FILE}"
+    if grep -q "#ifndef XRPL_" "${FILE}"; then
+      echo "Please replace all include guards by #pragma once."
+      exit 1
+    fi
+done
+echo "Checking complete."

.github/workflows/reusable-check-rename.yml

@@ -31,6 +31,8 @@ jobs:
         run: .github/scripts/rename/namespace.sh .
       - name: Check config name
         run: .github/scripts/rename/config.sh .
+      - name: Check include guards
+        run: .github/scripts/rename/include.sh .
       - name: Check for differences
         env:
           MESSAGE: |