Include quic_str_cat.h in quic_packet_creator.cc.
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

google/quiche,google/quiche,google/quiche,google/quiche