The modules fail to build with error: dereferencing pointer to incomplete type in r0drv sections for a few modules. Fix by adding the include #include <linux/sched.h>
to, /usr/src/vboxdrv-3.0.8/r0drv/linux/the-linux-kernel.h /usr/src/vboxnetadp-3.0.8/r0drv/linux/the-linux-kernel.h /usr/src/vboxnetflt-3.0.8/r0drv/linux/the-linux-kernel.h