C++ modernize: Pragma once (#6264)

* Migrate cpp headers to pragma once

src/httpfetch.h

@@ -17,8 +17,7 @@ with this program; if not, write to the Free Software Foundation, Inc.,
 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
 */
 
-#ifndef HTTPFETCH_HEADER
-#define HTTPFETCH_HEADER
+#pragma once
 
 #include <vector>
 #include "util/string.h"
@@ -113,5 +112,3 @@ void httpfetch_caller_free(unsigned long caller);
 // Performs a synchronous HTTP request. This blocks and therefore should
 // only be used from background threads.
 void httpfetch_sync(const HTTPFetchRequest &fetch_request, HTTPFetchResult &fetch_result);
-
-#endif // !HTTPFETCH_HEADER