add declaration for SerializeFloat

BUG=webm:1584

Change-Id: Ifa4dd5f0ef8ca4f320e521da94b61649a0d719eb
diff --git a/mkvmuxer/mkvmuxerutil.h b/mkvmuxer/mkvmuxerutil.h
index 132388d..3355428 100644
--- a/mkvmuxer/mkvmuxerutil.h
+++ b/mkvmuxer/mkvmuxerutil.h
@@ -31,6 +31,9 @@
 // Writes out |value| in Big Endian order. Returns 0 on success.
 int32 SerializeInt(IMkvWriter* writer, int64 value, int32 size);
 
+// Writes out |f| in Big Endian order. Returns 0 on success.
+int32 SerializeFloat(IMkvWriter* writer, float f);
+
 // Returns the size in bytes of the element.
 int32 GetUIntSize(uint64 value);
 int32 GetIntSize(int64 value);