HBASE-27361 Add .flattened-pom.xml to .gitignore (#4769)
HBASE-27361 Add .flattened-pom.xml to .gitignore (#4769)

Signed-off-by: Duo Zhang <ca823f7b4f21a31b6ef7892c1f90d53103f0d135@apache.org>
Apache9/hbase,mahak/hbase,Apache9/hbase,mahak/hbase,ndimiduk/hbase,Apache9/hbase,ndimiduk/hbase,Apache9/hbase,mahak/hbase,ndimiduk/hbase,mahak/hbase,mahak/hbase,ndimiduk/hbase,Apache9/hbase,Apache9/hbase,ndimiduk/hbase,mahak/hbase,Apache9/hbase,ndimiduk/hbase,mahak/hbase,mahak/hbase,ndimiduk/hbase,Apache9/hbase,mahak/hbase,ndimiduk/hbase,mahak/hbase,ndimiduk/hbase,Apache9/hbase,ndimiduk/hbase,Apache9/hbase