libbacktrace_libc++ needs to be multilib on the Mac too.

Change-Id: I2053345253c1d57c3485b790163ba0b4b10cc326
diff --git a/libbacktrace/Android.mk b/libbacktrace/Android.mk
index 1de81e7..c321369 100755
--- a/libbacktrace/Android.mk
+++ b/libbacktrace/Android.mk
@@ -201,6 +201,8 @@
 LOCAL_SRC_FILES := \
 	BacktraceMap.cpp \
 
+LOCAL_MULTILIB := both
+
 include $(BUILD_HOST_SHARED_LIBRARY)
 
 endif # TARGET_BUILD_APPS