http: use const http_header_t in stub

In preparation of libhtp rust

src/detect-http-headers-stub.h

@@ -57,7 +57,7 @@ static InspectionBuffer *GetRequestData(DetectEngineThreadCtx *det_ctx,
         if (htp_tx_request_headers(tx) == NULL)
             return NULL;
 
-        htp_header_t *h = (htp_header_t *)htp_tx_request_header(tx, HEADER_NAME);
+        const htp_header_t *h = htp_tx_request_header(tx, HEADER_NAME);
         if (h == NULL || htp_header_value(h) == NULL) {
             SCLogDebug("HTTP %s header not present in this request",
                        HEADER_NAME);
@@ -112,7 +112,7 @@ static InspectionBuffer *GetResponseData(DetectEngineThreadCtx *det_ctx,
         if (htp_tx_response_headers(tx) == NULL)
             return NULL;
 
-        htp_header_t *h = (htp_header_t *)htp_tx_response_header(tx, HEADER_NAME);
+        const htp_header_t *h = htp_tx_response_header(tx, HEADER_NAME);
         if (h == NULL || htp_header_value(h) == NULL) {
             SCLogDebug("HTTP %s header not present in this request",
                        HEADER_NAME);