Remove libbacktrace definition of gettid.

Also simplifies the Darwin implementation of gettid, because
apparently libbacktrace had a better way of doing it.

Bug: 19517541

(cherry picked from commit 23f750b068ddf5b20fb5d153a798b22562368992)

Change-Id: I2f888e8ed7a2f5719973786cbcbb399a81867ee9
diff --git a/libbacktrace/Backtrace.cpp b/libbacktrace/Backtrace.cpp
index d75c210..4e4003e 100644
--- a/libbacktrace/Backtrace.cpp
+++ b/libbacktrace/Backtrace.cpp
@@ -27,6 +27,8 @@
 #include <backtrace/Backtrace.h>
 #include <backtrace/BacktraceMap.h>
 
+#include <cutils/threads.h>
+
 #include "BacktraceLog.h"
 #include "thread_utils.h"
 #include "UnwindCurrent.h"