Dockerfile: add libc6-dev-i386 & gcc-multilib

Fixes the following error on travis-ci:
  CC       arch/x86/syscalls/syscall32.o
In file included from /usr/include/signal.h:28:0,
                 from /home/travis/build/0x7f454c46/criu/criu/arch/x86/include/asm/types.h:5,
                 from arch/x86/syscalls/syscall32.c:1:
/usr/include/features.h:374:25: fatal error: sys/cdefs.h: No such file or directory
 #  include <sys/cdefs.h>
                         ^
Signed-off-by: Dmitry Safonov <dsafonov@virtuozzo.com>
Acked-by: Cyrill Gorcunov <gorcunov@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>

scripts/build/Dockerfile.x86_64.hdr

 FROM ubuntu:trusty
+
+RUN apt-get update -qq && apt-get install -qq \
+                gcc-multilib \
+                libc6-dev-i386

scripts/travis/travis-tests

@@ -3,7 +3,8 @@ set -x -e
 
 TRAVIS_PKGS="protobuf-c-compiler libprotobuf-c0-dev libaio-dev
 		libprotobuf-dev protobuf-compiler python-ipaddr libcap-dev
-		libnl-3-dev gdb bash python-protobuf libnet-dev util-linux"
+		libnl-3-dev gcc-multilib libc6-dev-i386 gdb bash python-protobuf
+		libnet-dev util-linux"
 
 travis_prep () {
 	[ -n "$SKIP_TRAVIS_PREP" ] && return