update astxs to default /usr/src/asterisk

git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@2829 65c4cc65-6c06-0410-ace0-fbb531ad65f3

contrib/scripts/astxs

 #!/usr/bin/perl
 
-my $astdir = $ENV{ASTSRC} or "/usr/src/asterisk";
+my $astdir = $ENV{ASTSRC} || "/usr/src/asterisk";
 
 
 sub esystem($) {