driver.h: add declaration of log_src_driver_queue_method() to fix compilation warnings

The function log_src_driver_queue_method() was undeclared, causing
warnings to be displayed during compilation.

Signed-off-by: Balazs Scheidler <bazsi@balabit.hu>
Merge pull request #222 from balabit/h/fix-log-src-driver-queue-warning

driver.h: add declaration of log_src_driver_queue_method() to fix compil...