keepalived: needs headers >= 3.4

keepalived fails to build on toolchains with headers older than 3.4,
because of a namespace clash between the xt_set.h header from the
kernel and the linux_ip_set.h header installed by ipset.

Even though keepalived does check for pre-3.4 headers, the check
somehow fails to work correctly.

We fix that by making keepalived depend on headers 3.4 or later.

Fixes:
http://autobuild.buildroot.org/results/770/770d8fd2f3bacbdbe233da1b4d6e64e20a84d5a5/
http://autobuild.buildroot.org/results/1ad/1adb710b915427f681eae37452a0942833ce533e/
http://autobuild.buildroot.org/results/70b/70b31547e51ec7213372d2ef07bec34c5df77560/


[...]

Signed-off-by: Ilias Apalodimas <apalos@gmail.com>
Reviewed-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>