Hide ScopedTrace.

(cherry-pick of f2c1e7ee78a167ff323b9f45d20532d064d6778d.)

Bug: 11156955

(cherry picked from commit ebb6b4a4d3)

Change-Id: Ifc10364b35384a9f34e4c2c634dd78a16065a817

libc/private/bionic_systrace.h

@@ -23,7 +23,7 @@
 //   ScopedTrace("Trace message");
 // The trace will end when the contructor goes out of scope.
 
-class ScopedTrace {
+class __LIBC_HIDDEN__ ScopedTrace {
  public:
   explicit ScopedTrace(const char* message);
   ~ScopedTrace();