make: let src/Makefile set *dir vars properly
make: let src/Makefile set *dir vars properly

prefix suffice for setting *dir variables in src/Makefile; this does
away with setting does in in ./Makefile

Signed-off-by: Paymon MARANDI <c3b8163f22cbe48b549d9e7c13ceff28ab732d7b@gmail.com>

axboe/liburing,axboe/liburing,axboe/liburing