docs: Do not seek comments in kernel/rcu/tree_plugin.h

There are no kerneldoc comments in this file, so do not attempt to
include them in the docs build.

Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/driver-api/basics.rst

@@ -115,9 +115,6 @@ Kernel utility functions
 .. kernel-doc:: kernel/rcu/tree.c
    :export:
 
-.. kernel-doc:: kernel/rcu/tree_plugin.h
-   :export:
-
 .. kernel-doc:: kernel/rcu/update.c
    :export: