crimson/os/alienstore/CMakeLists: move alien::cflags

crimson/os/alienstore/CMakeLists: move alien::cflags
alien::cflags is now only relevant to crimson-alienstore.

Remove empty INTERFACE_COMPILE_DEFINITIONS.

Note: alien::cflags can be probably entirely removed and
replaced by target_include_directories. However, this causes
```
'seastar::continuation_base<>' has virtual functions but non-virtual destructor
```
for alien_*.cc users.
Can be addressed seperatly.

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