gstreamer: fix bison bump build failure

It's due to deprecations in bison 3.0+, fixes:
http://autobuild.buildroot.net/results/df4/df473cb34c9914b2c3d3d8ae344cbf7c19255468/



Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>