Don't depend on a phony target

Phony targets are always dirty, depend on the ndk.timestamp file instead
of the ndk phony rule.

Change-Id: I8a90e61a1a79861af59f66f3bc0fdeecd0819992

core/binary.mk

@@ -71,7 +71,7 @@ ifdef LOCAL_SDK_VERSION
   endif
 
   # Make sure we've built the NDK.
-  my_additional_dependencies += ndk
+  my_additional_dependencies += $(SOONG_OUT_DIR)/ndk.timestamp
 
   # mips32r6 is not supported by the NDK. No released NDK contains these
   # libraries, but the r10 in prebuilts/ndk had a local hack to add them :(