strace-graph: print the pid in the graph

* strace-graph: Prefix each command with its pid.
diff --git a/strace-graph b/strace-graph
index 680eb5f..5435e86 100755
--- a/strace-graph
+++ b/strace-graph
@@ -314,10 +314,10 @@
 	if ($$elem[0] eq 'EXEC') {
 	    my $argv = $$elem[2];
 	    if (defined $elapsed) {
-		print "$lead [$elapsed] @$argv\n";
+		print "$lead [$elapsed] $pid @$argv\n";
 		undef $elapsed;
 	    } else {
-		print "$lead @$argv\n";
+		print "$lead $pid @$argv\n";
 	    }
 	} elsif ($$elem[0] eq 'FORK') {
 	    if ($i == 1) {