Fix libbacktrace host build.

On host build, libbacktrace relies on libcutils to provide gettid()
function.

Change-Id: Iad83384ea699ca024b71b73cef26755287c9e6fe
diff --git a/libbacktrace/thread_utils.h b/libbacktrace/thread_utils.h
index df83581..9590657 100644
--- a/libbacktrace/thread_utils.h
+++ b/libbacktrace/thread_utils.h
@@ -19,6 +19,10 @@
 
 #include <unistd.h>
 
+#if !defined(__ANDROID__)
+#include <cutils/threads.h>
+#endif
+
 __BEGIN_DECLS
 
 int tgkill(int tgid, int tid, int sig);