cmake: do not include global_context.cc multiple times

cmake: do not include global_context.cc multiple times

otherwise ODR is violated:

==449025==ERROR: AddressSanitizer: odr-violation (0x000000f03700):
  [1] size=8 'g_ceph_context' ../src/global/global_context.cc:24:14
  [2] size=8 'g_ceph_context' ../src/global/global_context.cc:24:14
These globals were registered at these points:
  [1]:
    #0 0x4779bd in __asan_register_globals (/var/ssd/ceph/clang-build/bin/ceph-conf+0x4779bd)
    #1 0x56e9cb in asan.module_ctor (/var/ssd/ceph/clang-build/bin/ceph-conf+0x56e9cb)

  [2]:
    #0 0x4779bd in __asan_register_globals (/var/ssd/ceph/clang-build/bin/ceph-conf+0x4779bd)
    #1 0x7fe5fed12aeb in asan.module_ctor (/var/ssd/ceph/clang-build/lib/libceph-common.so.2+0x2f34aeb)

==449025==HINT: if you don't care about these errors you may set ASAN_OPTIONS=detect_odr_violation=0

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