package/systemd-bootchart: fix build w/ glibc 2.26

Fixes:
    http://autobuild.buildroot.org/results/7f7af0d41120626edbbca839c9fdbb5c6bd259c2/



Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Maxime Hadjinlian <maxime.hadjinlian@gmail.com>
Cc: Romain Naour <romain.naour@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>