[Refactor] Replace '#ifndef' with '#pragma once' (#9456)

* Replace '#ifndef' with '#pragma once'

be/src/runtime/string_buffer.hpp

@@ -15,8 +15,7 @@
 // specific language governing permissions and limitations
 // under the License.
 
-#ifndef DORIS_BE_SRC_QUERY_BE_RUNTIME_STRING_BUFFER_H
-#define DORIS_BE_SRC_QUERY_BE_RUNTIME_STRING_BUFFER_H
+#pragma once
 
 #include "runtime/mem_pool.h"
 #include "runtime/string_value.h"
@@ -107,5 +106,3 @@ private:
 };
 
 } // namespace doris
-
-#endif