ndk-gdb.py: fix adb forward path.

It turns out that forwarding a port to '/data/local/tmp\debug_socket'
doesn't work.

Bug: #46
Change-Id: I7d511681f51fc5c4f4672b670a0e40932031477d
(cherry picked from commit cdb472c)

Author: jmgao

Diff for: ndk-gdb.py

@@ -655,7 +655,7 @@ def main():
         zygote_path = os.path.join(out_dir, "system", "bin", "app_process")
 
     # Start gdbserver.
-    debug_socket = os.path.join(app_data_dir, "debug_socket")
+    debug_socket = posixpath.join(app_data_dir, "debug_socket")
     log("Starting gdbserver...")
     gdbrunner.start_gdbserver(
         device, None, gdbserver_path,