aidl: Don't link librt on Mac

This fixes unittest build on Mac.

Change-Id: I5180f647d1b5b41f99c4d0423aaec9d05fee6355
diff --git a/tools/aidl/Android.mk b/tools/aidl/Android.mk
index 354563a..542f272 100644
--- a/tools/aidl/Android.mk
+++ b/tools/aidl/Android.mk
@@ -64,7 +64,8 @@
     libgmock_host \
     libgtest_host \
 
-LOCAL_LDLIBS := -lrt
+LOCAL_LDLIBS_linux := -lrt
+
 include $(BUILD_HOST_NATIVE_TEST)
 
 endif # No TARGET_BUILD_APPS or TARGET_BUILD_PDK