Support compiling libgui against musl

Support compiling libgui against musl by including the missing stdard.h
header.

Bug: 190084016
Test: m USE_HOST_MUSL=true host-native
Change-Id: I35afcf47ed73c64d4fdad2e143a9bf71ec3973fb
diff --git a/libs/gui/include/gui/TraceUtils.h b/libs/gui/include/gui/TraceUtils.h
index b9ec14a..e5d2684 100644
--- a/libs/gui/include/gui/TraceUtils.h
+++ b/libs/gui/include/gui/TraceUtils.h
@@ -16,6 +16,8 @@
 
 #pragma once
 
+#include <stdarg.h>
+
 #include <cutils/trace.h>
 #include <utils/Trace.h>