Update INSTALL: hslogger is mandatory
Update INSTALL: hslogger is mandatory

hslogger is a mandatory Haskell dependency form 2.9 onwards. Hence
update the INSTALL instructions accordingly.

Signed-off-by: Klaus Aehlig <4ce51de36d9068cd7db2eec1151e0f3f95419008@google.com>
Reviewed-by: Thomas Thrainer <98bdeac947eb8ca18146e9ce7f092788ec242cd1@google.com>

dimara/ganeti,andir/ganeti,leshchevds/ganeti,grnet/snf-ganeti,onponomarev/ganeti,apyrgio/snf-ganeti,apyrgio/ganeti,andir/ganeti,dimara/ganeti,mbakke/ganeti,yiannist/ganeti,leshchevds/ganeti,leshchevds/ganeti,yiannist/ganeti,ganeti-github-testing/ganeti-test-1,mbakke/ganeti,ganeti-github-testing/ganeti-test-1,onponomarev/ganeti,apyrgio/snf-ganeti,apyrgio/ganeti,bitemyapp/ganeti,ganeti/ganeti,mbakke/ganeti,grnet/snf-ganeti,ganeti/ganeti,yiannist/ganeti,ganeti/ganeti,bitemyapp/ganeti,andir/ganeti