cmake: do not use plain target_link_libraries(rgw_a ...)

cmake: do not use plain target_link_libraries(rgw_a ...)

this addresses following error:

CMake Error at src/rgw/CMakeLists.txt:178 (target_link_libraries):
  The plain signature for target_link_libraries has already been used
with
  the target "rgw_a".  All uses of target_link_libraries with a target
must
  be either all-keyword or all-plain.

  The uses of the plain signature are here:

   * src/rgw/CMakeLists.txt:168 (target_link_libraries)

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