Install libffi6 on dpkg platforms

We need the library, not the dev library, in the final
locations.

Change-Id: I944f8ce184836ca7c1b99727b481f338f7686090

bindep.txt

@@ -3,8 +3,8 @@
 
 gcc [compile test]
 libffi-devel [platform:rpm]
-libffi-dev [platform:dpkg]
-libffi-dev [compile test platform:apk]
+libffi-dev [compile test platform:apk platform:dpkg]
+libffi6 [platform:dpkg]
 libffi [platform:apk]
 libressl-dev [compile test platform:apk]
 linux-headers [compile test platform:apk]