[infer][PR] Dockerfile: add missing libffi build dependency

Summary:
Fixes this error in the Docker image build (mentioned in https://github.com/facebook/infer/issues/570#issuecomment-282191663):

```
#=== ERROR while installing ctypes.0.11.3 =====================================#
# opam-version         1.2.2
# os                   linux
# command              make XEN=disable ctypes-foreign
# path                 /root/.opam/infer-4.02.3/build/ctypes.0.11.3
# compiler             4.02.3
# exit-code            2
# env-file             /root/.opam/infer-4.02.3/build/ctypes.0.11.3/ctypes-5557-6ff7fe.env
# stdout-file          /root/.opam/infer-4.02.3/build/ctypes.0.11.3/ctypes-5557-6ff7fe.out
# stderr-file          /root/.opam/infer-4.02.3/build/ctypes.0.11.3/ctypes-5557-6ff7fe.err
### stdout ###
# [...]
# The following required C libraries are missing: libffi.
# Please install them and retry. If they are installed in a non-standard location
# or need special flags, set the environment variables <LIB>_CFLAGS and <LIB>_LIBS
# accordingly and retry.
#
#  For e
Closes https://github.com/facebook/infer/pull/587

Reviewed By: mbouaziz

Differential Revision: D4605385

Pulled By: jvillard

fbshipit-source-id: 689cd4e

docker/Dockerfile

@@ -19,6 +19,7 @@ RUN apt-get update && \
             git \
             groff \
             libc6-dev \
+            libffi-dev \
             libgmp-dev \
             libmpc-dev \
             libmpfr-dev \