mesa3d: fix build with BR2_SHARED_STATIC_LIBS

mesa3d does not allow to enable both static and shared libraries so if
BR2_SHARED_STATIC_LIBS is set, disable static

Fixes https://bugs.busybox.net/show_bug.cgi?id=10326



Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>