cmake: bump liburing from 0.7 to 2.5

cmake: bump liburing from 0.7 to 2.5

this allows us to use newer liburing features. Seastar is using
some of them which are not provided by liburing 0.7.

in this change, `--use-libc` is passed to configure. otherwise
it does not link against libc, and the symbles like memset()
won't be available when compiling liburing.so with -fPIC using
clang, which does not pull libc in that case.

Signed-off-by: Kefu Chai <tchaikov@gmail.com>