MINOR: mux-h2: add missing glitch count for non-decodable H2 headers

One rare error case could produce a protocol error on the stream when
not being able to decode response headers wasn't being accounted as a
glitch, so let's fix it.

src/mux_h2.c

@@ -3721,6 +3721,7 @@ static struct h2s *h2c_bck_handle_headers(struct h2c *h2c, struct h2s *h2s)
 		}
 
 		/* stream error : send RST_STREAM */
+		h2c_report_glitch(h2c, 1, "couldn't decode response HEADERS");
 		TRACE_ERROR("couldn't decode response HEADERS", H2_EV_RX_FRAME|H2_EV_RX_HDR, h2c->conn, h2s);
 		h2s_error(h2s, H2_ERR_PROTOCOL_ERROR);
 		h2c->st0 = H2_CS_FRAME_E;