Add LOCAL_MULTILIB := both to Darwin libbacktrace.

This was previously set for libbacktrace_libc++, but apparently never
was for libbacktrace.

Change-Id: I85dc44b356710c6367f5cea3bc0a4d6c817ca202
diff --git a/libbacktrace/Android.mk b/libbacktrace/Android.mk
index e2c0a4d..01b4f4d 100755
--- a/libbacktrace/Android.mk
+++ b/libbacktrace/Android.mk
@@ -141,6 +141,8 @@
 LOCAL_SRC_FILES := \
 	BacktraceMap.cpp \
 
+LOCAL_MULTILIB := both
+
 include $(BUILD_HOST_SHARED_LIBRARY)
 
 endif # HOST_OS-darwin