Prevent include failures of quic_flag_utils

We've had quiche merge failures due to missing includes because in google3 quic_flags ends up including quic_flag_utils but not in Chromium. This ensures inclusion in the api header to make sure behavior is consistent between google3 and Chromium. This is also useful because it is very common for code that uses flags to also want to use flag counts.

gfe-relnote: n/a, compile-only
PiperOrigin-RevId: 248687080
Change-Id: I0a163bb06d4b4a8ecc1747245214f329ba565ba3
diff --git a/quic/platform/api/quic_flags.h b/quic/platform/api/quic_flags.h
index 302036f..bf6fc3c 100644
--- a/quic/platform/api/quic_flags.h
+++ b/quic/platform/api/quic_flags.h
@@ -8,6 +8,7 @@
 #include <string>
 #include <vector>
 
+#include "net/third_party/quiche/src/quic/platform/api/quic_flag_utils.h"
 #include "net/quic/platform/impl/quic_flags_impl.h"
 
 // Define a command-line flag that can be automatically set via