ITS#9511 - Stop plugin.c from generating an error when running make depend...

ITS#9511 - Stop plugin.c from generating an error when running make depend even if slapi is not enabled
3 jobs for master in 43 minutes and 11 seconds (queued for 1 second)
Name Stage Failure
failed
build-openssl-heimdal-lloadd Build
make[1]: *** [Makefile:285: test] Error 2
make[1]: Leaving directory '/builds/openldap/openldap/tests'
make: *** [Makefile:297: test] Error 2
Uploading artifacts for failed job
Uploading artifacts...
tests/testrun/: found 94 matching files and directories

Uploading artifacts as "archive" to coordinator... ok
id=6168 responseStatus=201 Created token=EZTXrosB
Cleaning up file based variables
ERROR: Job failed: exit code 1