Make use of -lppapi explicit to allow for correct library order.

native_client_sdk/src/examples/mouselock/example.dsc

 {
   'TOOLS': ['newlib', 'glibc', 'win'],
   'TARGETS': [
     {
       'NAME' : 'mouselock',
       'TYPE' : 'main',
       'SOURCES' : ['mouselock.cc', 'mouselock.h'],
-      'LIBS': ['ppapi_cpp']
+      'LIBS': ['ppapi_cpp', 'ppapi', 'pthread']
     }
   ],
   'DATA': ['check_browser.js'],
   'DEST': 'examples',
   'NAME': 'mouselock',
   'TITLE': 'Mouse Lock',
   'DESC': """
 The Mouselock example demonstrates how to use the MouseLock API to hide
 the mouse cursor.  Mouse lock is only available in full-screen mode.  You can
 lock and unlock the mouse while in full-screen mode by pressing the Enter key.
 """,
   'INFO': 'Teaching focus: Mouse lock, Full-screen.'
 }