Include quic_str_cat.h in quic_packet_creator.cc.

This is for QuicStrCat in line 847.  Chromium build fails without this include
after merging some recent CLs.  Presumably it is transitively incuded by some
platform/impl in Google3.

gfe-relnote: n/a.  Include header already transitively included.
PiperOrigin-RevId: 246511319
Change-Id: Ifb6f92270aada7c9fb676c6969493c30b1e5004b

quic/core/quic_packet_creator.cc

diff --git a/quic/core/quic_packet_creator.cc b/quic/core/quic_packet_creator.cc
index 4934666..f3390f0 100644
--- a/quic/core/quic_packet_creator.cc
+++ b/quic/core/quic_packet_creator.cc
@@ -22,6 +22,7 @@
 #include "net/third_party/quiche/src/quic/platform/api/quic_flags.h"
 #include "net/third_party/quiche/src/quic/platform/api/quic_logging.h"
 #include "net/third_party/quiche/src/quic/platform/api/quic_ptr_util.h"
+#include "net/third_party/quiche/src/quic/platform/api/quic_str_cat.h"
 #include "net/third_party/quiche/src/quic/platform/api/quic_string_piece.h"
 #include "net/third_party/quiche/src/quic/platform/api/quic_text_utils.h"