Add a missing $, for interpreting data as a variable

This fixes ndk-build clean in projects that e.g. include
prebuilt static libraries, such as stlport.

Change-Id: I7d8d547e8b481db240cad5802d39f26f595c786a
diff --git a/build/core/build-binary.mk b/build/core/build-binary.mk
index e6710f2..ebdf15b 100644
--- a/build/core/build-binary.mk
+++ b/build/core/build-binary.mk
@@ -46,7 +46,7 @@
 $(cleantarget): PRIVATE_CLEAN_FILES := $(LOCAL_BUILT_MODULE) \
                                        $($(my)OBJS)
 else
-$(cleantarget): PRIVATE_CLEAN_FILES := ($(my)OBJS)
+$(cleantarget): PRIVATE_CLEAN_FILES := $($(my)OBJS)
 endif
 $(cleantarget)::
 	@$(HOST_ECHO) "Clean: $(PRIVATE_MODULE) $(PRIVATE_TEXT)"