Add libffi-dev package

This is required for pyOpenSSL to build (via pip).

The errors presented are:


No package 'libffi' found

compiling '_configtest.c':

__thread int some_threadlocal_variable_42;



gcc -pthread -fno-strict-aliasing -DNDEBUG -g -fwrapv -O2 -Wall -Wstrict-prototypes -fPIC -c _configtest.c -o _configtest.o

success!

removing: _configtest.c _configtest.o

c/_cffi_backend.c:13:17: fatal error: ffi.h: No such file or directory