gdbclient: fix darwin sed problem

darwin does not takes \t for sed as tab.
Use [[:blank:]] instead

Change-Id: Ic6c08ceacf3918eb0da52a200c03e035c77a88ee
diff --git a/scripts/gdbclient b/scripts/gdbclient
index 19a6949..acbbdda 100755
--- a/scripts/gdbclient
+++ b/scripts/gdbclient
@@ -24,7 +24,7 @@
 
 # returns 0 when process is not traced
 function adb_get_traced_by() {
-  echo `adb shell cat /proc/$1/status | grep -e "^TracerPid:" | sed "s/^TracerPid:\t//" | tr -d '\r\n'`
+  echo `adb shell cat /proc/$1/status | grep -e "^TracerPid:" | sed "s/^TracerPid:[[:blank:]]//" | tr -d '\r\n'`
 }
 
 function get_symbols_directory()