[klibc] Makefile: Update help text for KLIBCKERNELSRC

Since the UAPI/KAPI header split in Linux 3.7, we have needed
KLIBCKERNELSRC to point to the installed UAPI headers.  The "sample
invocation" has been updated to reflect this but the rest of the help
text has not.

Link: https://www.zytor.com/pipermail/klibc/2019-January/004032.html
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
1 file changed