package/mesa3d-demos: Fix build error

Fixes
http://autobuild.buildroot.net/results/8f2/8f292f1151089d20f33c599320941cb5435c0199/
http://autobuild.buildroot.net/results/f3d/f3db50d0c5c74fecafd6b4bdd89f97e8e566d820/
http://autobuild.buildroot.net/results/ac0/ac0f22bf87e2b48bd2191de88999404d1c55de9b/
http://autobuild.buildroot.net/results/da8/da87a3e81b19ad038661f5653b544f4c8cfd389d/
http://autobuild.buildroot.net/results/d51/d51d22754a37f790d2de5a4836237b8eea417124/
http://autobuild.buildroot.net/results/347/347b2f79111548ae00f40879c1b5f3c4c1c7091e/
http://autobuild.buildroot.net/results/aef/aef73a3bc770c3784025642f6a2c276944117112/
http://autobuild.buildroot.net/results/901/90152f1cfa09ad4d37700b340204f8b75850c582/



Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>