srcu: Make SRCU be built by default

SRCU is optional, and included only if there is a "select SRCU" in effect.
However, we now have Tiny SRCU, so this commit defaults CONFIG_SRCU=y.

Reported-by: kbuild test robot <fengguang.wu@intel.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>