package/capnproto: require GCC 5 for C++14

package/capnproto: require GCC 5 for C++14

Adds dependency on at least GCC 5 to have C++14 language features that
are required starting in version 0.7.0 of capnproto.

Fixes:
http://autobuild.buildroot.org/results/5c09e745cab822d830f73e33647f3b0e765c9181
(capnproto build failure)

Fixes:
http://autobuild.buildroot.org/results/743c750e9932658c20965a25de89c3f21a1d43e9
(host-capnproto build failure)

This updated dependency is propagated to capnproto unique reverse
dependency, c-capnproto.

Signed-off-by: Koen Martens <gmc@sonologic.nl>
Signed-off-by: Joel Carlson <JoelsonCarl@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>