hidl-gen: Fix -Lmakefile-impl.

Bug: 31055568

Test: hidl-gen -o system/tools/hidl/fooimpl/ -Lmakefile-impl
        -randroid.hardware:hardware/interfaces
        android.hardware.tests.foo@1.0

Change-Id: I35c550aa443bffb0c36e1006d563e5937837c11c
diff --git a/main.cpp b/main.cpp
index b81f004..28a6b8f 100644
--- a/main.cpp
+++ b/main.cpp
@@ -896,7 +896,7 @@
     },
 
     {"makefile-impl",
-     OutputHandler::NOT_NEEDED /* mOutputMode */,
+     OutputHandler::NEEDS_DIR /* mOutputMode */,
      validateForMakefile,
      generateMakefileImplForPackage,
     }