coredumpctl: fix build with !HAVE_XZ

Fixes the following build error:

  CCLD     coredumpctl
src/journal/coredumpctl.o: In function `save_core':
/src/systemd-master/src/journal/coredumpctl.c:656:
undefined reference to `decompress_stream'
collect2: error: ld returned 1 exit status
make[2]: *** [coredumpctl] Error 1
make[1]: *** [all-recursive] Error 1
make: *** [all] Error 2
coredumpctl: fix HAVE_XZ check