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/Semaphore.h b/libmemunreachable/Semaphore.h
index 6d39a93..cd73972 100644
--- a/libmemunreachable/Semaphore.h
+++ b/libmemunreachable/Semaphore.h
@@ -22,6 +22,8 @@
 
 #include "android-base/macros.h"
 
+namespace android {
+
 class Semaphore {
  public:
   explicit Semaphore(int count = 0) : count_(count) {}
@@ -53,4 +55,6 @@
   std::condition_variable cv_;
 };
 
+}  // namespace android
+
 #endif  // LIBMEMUNREACHABLE_SEMAPHORE_H_