lang/mono6.8: Fix build with archivers/brotli

Add USES=localbase, so in-tree headers are found before headers in
LOCALBASE. This fixes the build with archivers/brotli installed.

CONFLICTS isn't an option any more, after
478e294ddedbe6444330b00512a09406265bea71, archivers/brotli is an
indirect build dependency.

PR: 265745
Approved by: portmgr (build fix blanket), tcberner (mentor)