_sd-common.h: avoid parsing errors with Coverity

The commit 1070d271 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."
     ^