Add a missing QUIC_EXPORT in http_constants.

PiperOrigin-RevId: 353691832
Change-Id: Ib3a0288247eeeb7ddba801eabc45efa5b96e42fa

quic/core/http/http_constants.h

diff --git a/quic/core/http/http_constants.h b/quic/core/http/http_constants.h
index 65096a5..8c68e74 100644
--- a/quic/core/http/http_constants.h
+++ b/quic/core/http/http_constants.h
@@ -9,6 +9,7 @@
 #include <string>
 
 #include "quic/core/quic_types.h"
+#include "quic/platform/api/quic_export.h"
 
 namespace quic {
 
@@ -36,7 +37,8 @@
 };
 
 // Returns HTTP/3 SETTINGS identifier as a string.
-std::string H3SettingsToString(Http3AndQpackSettingsIdentifiers identifier);
+QUIC_EXPORT std::string H3SettingsToString(
+    Http3AndQpackSettingsIdentifiers identifier);
 
 // Default maximum dynamic table capacity, communicated via
 // SETTINGS_QPACK_MAX_TABLE_CAPACITY.