Add ramdisk.img to the dist dir, again (was removed at some point)
b/2590492 and requested again

Change-Id: I5e4c8efddf7a202694c36231c6082089eedefa5d
diff --git a/core/main.mk b/core/main.mk
index eadb8fd..f1a9beb 100644
--- a/core/main.mk
+++ b/core/main.mk
@@ -720,6 +720,7 @@
     $(INSTALLED_BUILD_PROP_TARGET) \
     $(BUILT_TARGET_FILES_PACKAGE) \
     $(INSTALLED_ANDROID_INFO_TXT_TARGET) \
+    $(INSTALLED_RAMDISK_TARGET) \
    )
 
   # Tests are installed in userdata.img.  If we're building the tests