doc: Fix RCU's docbook options

Commit 764f80798b95 ("doc: Add RCU files to docbook-generation files")
added :external: options for RCU source files in the file
Documentation/core-api/kernel-api.rst.  However, this now means
nothing, so this commit changes them to ":export:".

Reported-by: Randy Dunlap <rdunlap@infradead.org>
Reported-by: Akira Yokosawa <akiyks@gmail.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
1 file changed