Update configure help msg to match actual defaults
The defaults for LINUX_KERNEL_PATH and LINUX_KERNEL_BUILD in
acinclude.m4 were changed in 2cfd611, 94ff565 and 3f9d982 without updating
the output of ./configure --help. Change the description of
linux-kernel-headers and linux-kernel-build to show the correct defaults.
Change-Id: I41331f4d48a555e291a45ca56e788bc418c064d3
Reviewed-on: http://gerrit.openafs.org/8759
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>