Include <cstdint> in quiche_platform_impl/quic_flags_impl.h

Many protocol flags have type uint32_t, int32_t, or int64_t.

Remove unnecessary <string> include.

This (hopefully) unblocks using the default implementation in Chromium.

PiperOrigin-RevId: 449264238

quiche/common/platform/default/quiche_platform_impl/quic_flags_impl.h

diff --git a/quiche/common/platform/default/quiche_platform_impl/quic_flags_impl.h b/quiche/common/platform/default/quiche_platform_impl/quic_flags_impl.h
index a7d9bb7..23758ca 100644
--- a/quiche/common/platform/default/quiche_platform_impl/quic_flags_impl.h
+++ b/quiche/common/platform/default/quiche_platform_impl/quic_flags_impl.h
@@ -5,7 +5,7 @@
 #ifndef QUICHE_COMMON_PLATFORM_DEFAULT_QUICHE_PLATFORM_IMPL_QUIC_FLAGS_IMPL_H_
 #define QUICHE_COMMON_PLATFORM_DEFAULT_QUICHE_PLATFORM_IMPL_QUIC_FLAGS_IMPL_H_
 
-#include <string>
+#include <cstdint>
 
 #include "quiche/common/platform/api/quiche_export.h"