package/mp4v2: fix build with gcc <= 5

Fixes:
 - http://autobuild.buildroot.org/results/14937c96a82fb3d10e5d83bd7b2905b846fb09f9



Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
[yann.morin.1998@free.fr: expand the patch' commit log]
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>