bootstrap: Remove rsync from buildreq list

rsync is used to download .po files, but SKIP_PO=true is set and
downloading .po files is skipped.

This also fixes a problem with MinGW builds, because rsync is not
available for MinGW.

bootstrap.conf

@@ -129,7 +129,6 @@ git        1.5.5
 gzip       -
 libtool    -
 perl       5.5
-rsync      -
 tar        -
 "