dockerrpm: fix CentOS 5 RPMs (issue4977) stable

dockerrpm: fix CentOS 5 RPMs (issue4977)

The older rpmbuild in CentOS 5 fails if some of the output directories
have not been created yet. This change results in creating those directories.

--- a/contrib/dockerrpm	Sat Jan 02 02:04:32 2016 +0100
+++ b/contrib/dockerrpm	Mon Jan 04 21:21:59 2016 +0100
@@ -15,6 +15,8 @@
 RPMBUILDDIR=$ROOTDIR/packages/$PLATFORM
 contrib/buildrpm --rpmbuilddir $RPMBUILDDIR --prepare $*
 
+mkdir -p $RPMBUILDDIR/{BUILD,SRPMS,RPMS}
+
 DSHARED=/mnt/shared
 $DOCKER run -u $DBUILDUSER --rm -v $RPMBUILDDIR:$DSHARED $CONTAINER \
     rpmbuild --define "_topdir $DSHARED" -ba $DSHARED/SPECS/mercurial.spec --clean