package/weston: fix build without EGL

Fixes:
    http://autobuild.buildroot.org/results/378/378ffddcebd0ab3dabfdb268614d1344a38e8dd1/
    http://autobuild.buildroot.org/results/3c2/3c25333d3c5838bea27540d8ac752c8bdf4db843/


    [...]

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>