crimson/os/alienstore/CMakeLists: Cleanup alien::cflags interface

crimson/os/alienstore/CMakeLists: Cleanup alien::cflags interface

alienstore FTBFS [1] due to virtual-dtor warning when compiling seastar [2].
Instead of using alien::cflags which define INTERFACE_COMPILE_OPTIONS of
-Wno-non-virtual-dtor - Let's directly add this compile option to
tagets using seastar.

Crimson non-alien targets solve that with crimson::cflags which
defines the relevant compile flag. However, we don't reuse it here since
it also carries WITH_CRIMSON.

As both crimson::cflags and crimson-alienstore which are using seastar
have to set no-non-virtual-dtor - The compile option moved to the common
cmake file instead of setting it in both targets.

[1]
```
crimson/os/alienstore/alien_log.cc:21:28:   required from here
seastar/include/seastar/core/future.hh:666:7:
warning: ‘class seastar::continuation_base<void>’ has virtual functions
and accessible non-virtual destructor [-Wnon-virtual-dtor]
```

[2] See 36963dd7ccd50eff2f8b91d97ffb66295a7a6292

Signed-off-by: Matan Breizman <mbreizma@redhat.com>