cmake: make rgw_common a static library

cmake: make rgw_common a static library

it was changed from a shared library to object library back in
3c0bba40b2fcc36a02bfe89647231e8983819a72, because we wanted to link both
rgw_a and rgw_admin_user against it.

but now that rgw_admin_user was dropped in
c89b59428e66ae6d89d1fd27a4280045c57251cb, let's compile rgw_common
as a static library. it'd be much easier for us to apply various compile
options and linkages to it this way.

Signed-off-by: Kefu Chai <kchai@redhat.com>