_sd-common.h: avoid parsing errors with Coverity

_sd-common.h: avoid parsing errors with Coverity

The commit 1070d271fa8fa553d57dd5f74dd1e3f60732d0b9 which was supposed
too fix this does not seem to take effect any more.  We get again 34%
compilation success rate while scanning systemd itself.  Moreover, the
installed header file breaks compilation of programs that include it:

"/usr/include/systemd/_sd-common.h", line 23: error #35: #error directive: "Do
          not include _sd-common.h directly; it is a private header."
  #  error "Do not include _sd-common.h directly; it is a private header."
     ^

(cherry picked from commit 4191b3282afbca9f1ef333f91bb6566c374da1fe)
(cherry picked from commit 5aec8fe54e47dbffc9ed705e4211f935bdca1550)
(cherry picked from commit 42f329455667e48131c2a8d3d63f5ce2211d9a92)