Move libmemunreachable into namespace android

Putting libmemunreachable in the global C++ namespace was an oversight,
move it into namespace android.

Test: m -j checkbuild
Change-Id: I0799906f6463178cb04a719bb4054cad33a50dbe
diff --git a/libmemunreachable/PtracerThread.h b/libmemunreachable/PtracerThread.h
index ddf9026..4f9c420 100644
--- a/libmemunreachable/PtracerThread.h
+++ b/libmemunreachable/PtracerThread.h
@@ -24,6 +24,8 @@
 
 #include "Allocator.h"
 
+namespace android {
+
 class Stack;
 
 // PtracerThread is similar to std::thread, except that it creates a "thread"
@@ -48,4 +50,6 @@
   pid_t child_pid_;
 };
 
+}  // namespace android
+
 #endif  // LIBMEMUNREACHABLE_PTRACER_THREAD_H_