add libffi-dev to DEB_DEPENDS
add libffi-dev to DEB_DEPENDS

Package is already in the RPM_DEPENDS list, and is required for test-* targets

Change-Id: I3846e68855402b50a51adf3daf218e0244562cd6
Signed-off-by: Gabriel Ganne <54ed6f2b99047a4f6b8431ec375373608f566aee@enea.com>

chrisy/vpp,FDio/vpp,chrisy/vpp,chrisy/vpp,FDio/vpp,chrisy/vpp,FDio/vpp,FDio/vpp,FDio/vpp,vpp-dev/vpp,vpp-dev/vpp,chrisy/vpp,vpp-dev/vpp,chrisy/vpp,vpp-dev/vpp,chrisy/vpp,chrisy/vpp,FDio/vpp,vpp-dev/vpp,vpp-dev/vpp,FDio/vpp,vpp-dev/vpp,FDio/vpp