external/mambo: Check for qtrace_utils.tcl before sourcing it

Commit cb835db ('external/mambo: conditionally source qtrace script')
added qtrace_utils.tcl sourcing in skiboot.tcl without a check to see
whether it exists in the current directory. This broke running mambo from
another directory using skiboot.tcl. Patch adds a check.

Fixes: cb835db ('external/mambo: conditionally source qtrace script')
Signed-off-by: Madhavan Srinivasan <maddy@linux.vnet.ibm.com>
Signed-off-by: Stewart Smith <stewart@linux.ibm.com>

external/mambo/skiboot.tcl

@@ -1,6 +1,8 @@
 # need to get images path defined early
 source $env(LIB_DIR)/ppc/util.tcl
-source qtrace_utils.tcl
+if { [file exists qtrace_utils.tcl] } then {
+	source qtrace_utils.tcl
+}
 
 #
 # Call tclreadline's Loop to move to friendlier