Makefile: add -Wformat-security to CFLAGS
Makefile: add -Wformat-security to CFLAGS

Apparently when travis checks patches, it compiles code with
-Wformat-security (most probably because the distro/gcc it uses
has it on by default), but on my system (Fedora 24/gcc 6.1.1)
this flag is not on. As a result, code compiles fine for me
but travis reports an error.

Add -Wformat-security to default CFLAGS. It helps to catch
problems like using printf(str) instead of printf("%s", str).

Signed-off-by: Kir Kolyshkin <c0159612b0ef32139322233232df1a0d189c4b9b@openvz.org>
Signed-off-by: Pavel Emelyanov <c9a32589e048e044184536f7ac71ef92fe82df3e@virtuozzo.com>

KKoukiou/criu-remote,KKoukiou/criu-remote,KKoukiou/criu-remote,KKoukiou/criu-remote,KKoukiou/criu-remote,KKoukiou/criu-remote