so we can have access to the Seastar_PRIVATE_CXX_FLAGS when
defining the `seastar-module` target. we will do this in another
commit.

Signed-off-by: Kefu Chai <[email protected]>

before this change, when building with C++20 modules enabled, some
symbols are not accessible, when compiling source files like
src/core/resource.cc if some features are enabled using macros like
SEASTAR_HAVE_HWLOC and SEASTAR_HAVE_URING.

in this change, we include the missing headers or move the include
precessor directives, so that the .cc files can have access to the
used declaration of the used symbols.

please note, this change not only enables us to build C++20 modules
with SEASTAR_HAVE_HWLOC and/or SEASTAR_HAVE_URING defined, it is also
necessary also from the correctness point of view.

Signed-off-by: Kefu Chai <[email protected]>

before this change, when building seastar-module, we don't check for
the enabled features. so, for instance io uring backend is not enabled
even `Seastar_IO_URING` is set.

in this change we

* check for the enabled options,
* link against the related libraries, and
* add the required macro definitions

this should enable seastar-module to build with, for instance, io_uring
backend enabled. as `SEASTAR_HAVE_URING` enables this backend when
building `src/core/reactor_backend.cc`.

Fixes scylladb#2249
Signed-off-by: Kefu Chai <[email protected]>