build: create include/xfs before installing headers

build: create include/xfs before installing headers

Currently the install-headers rule from include/Makefile creates
include/xfs, but there is no guarantee that it will be the first
directory that make executes that rule in. Hence other directories
can race with the creation on include/xfs and fail.

Move the creation of include/xfs to occur before running the
install_headers rules on the subdirectories to avoid any possible
races with creation.

Signed-off-by: Dave Chinner <dchinner@redhat.com>