acl: Make it compatible with externalsrc

acl: Make it compatible with externalsrc

Make the subdir fetch path for configure.ac relative. This avoids the
following error after having done `devtool modify acl`:

ERROR: acl-2.2.52-r0 do_unpack: Unpack failure for URL:
'file://configure.ac;subdir=.../builds/qemux86-64/workspace/sources/acl'.
subdir argument isn't a subdirectory of unpack root
.../builds/qemux86-64/tmp/work/core2-64-poky-linux/acl/2.2.52-r0

(From OE-Core rev: 8a12bb7349bce29403077e18875563a9b0770838)

Signed-off-by: Peter Kjellerstedt <peter.kjellerstedt@axis.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>