[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

Differential Revision: D4620091

Pulled By: jvillard

fbshipit-source-id: 14c95f4

info.json

@@ -0,0 +1,15 @@
+{
+    "_README_WHAT_IS_THIS_DIFF_": "This diff is temporary, and will be replaced once the \"convert-pull-request-to-internal-diff\" sandcastle job finishes. For more details, see https:\/\/fburl.com\/pr-import-on-sandcastle",
+    "repository": {
+        "fbid": 788847014501904,
+        "name": "facebook\/infer"
+    },
+    "pull_request": {
+        "fbid": 352268038506620,
+        "number": 587,
+        "name": "Dockerfile: add missing libffi build dependency",
+        "author": "3flex",
+        "initiator": "jvillard"
+    },
+    "head_sha": "1bc647221616bbc45c938bbddbcd810493c9c0a4"
+}

pull-request.patch

@@ -0,0 +1,21 @@
+From 1bc647221616bbc45c938bbddbcd810493c9c0a4 Mon Sep 17 00:00:00 2001
+From: Matthew Haughton <3flex@users.noreply.github.com>
+Date: Thu, 23 Feb 2017 22:24:10 -0500
+Subject: [PATCH] Dockerfile: add missing libffi build dependency
+
+---
+ docker/Dockerfile | 1 +
+ 1 file changed, 1 insertion(+)
+
+diff --git a/docker/Dockerfile b/docker/Dockerfile
+index 6c5f130..748ef4b 100644
+--- a/docker/Dockerfile
++++ b/docker/Dockerfile
+@@ -19,6 +19,7 @@ RUN apt-get update && \
+             git \
+             groff \
+             libc6-dev \
++            libffi-dev \
+             libgmp-dev \
+             libmpc-dev \
+             libmpfr-dev \