cmake: expose pmem::pmemobj from rbd_internal

cmake: expose pmem::pmemobj from rbd_internal

because `librbd/cache/rwl/Types.h` includes `libpmemobj.h`, without this
change, `cache/test_mock_ReplicatedWriteLog.cc` will fail to compile as
`libpmemobj.h` might not exist in the default directories compile search
for header files. and hence we could have following failure
```
ceph/src/test/librbd/cache/test_mock_ReplicatedWriteLog.cc:11:
86 /root/luyuan/community/github/ceph/src/librbd/cache/rwl/Types.h:8:10: fatal error: libpmemobj.h: No such file or directory
87 8 | #include <libpmemobj.h>
88 | ^~~~~~~~~~~~~~
89 compilation terminated.
90 src/test/librbd/CMakeFiles/unittest_librbd.dir/build.make:1814: recipe for target 'src/test/librbd/CMakeFiles/unittest_librbd.dir/cache/test_mock_ReplicatedWriteLog.cc.o' failed
91 make[3]: *** [src/test/librbd/CMakeFiles/unittest_librbd.dir/cache/test_mock_ReplicatedWriteLog.cc.o] Error 1
92 make[3]: *** Waiting for unfinished jobs....
93 CMakeFiles/Makefile2:20379: recipe for target 'src/test/librbd/CMakeFiles/unittest_librbd.dir/all' failed
94 make[2]: *** [src/test/librbd/CMakeFiles/unittest_librbd.dir/all] Error 2
95 CMakeFiles/Makefile2:20391: recipe for target 'src/test/librbd/CMakeFiles/unittest_librbd.dir/rule' failed
96 make[1]: *** [src/test/librbd/CMakeFiles/unittest_librbd.dir/rule] Error 2
97 Makefile:5375: recipe for target 'unittest_librbd' failed
```

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