AP_Proximity: mark many functions as __INITFUNC__

... implying they can go into external flash

libraries/AP_Proximity/AP_Proximity.h

@@ -102,7 +102,7 @@ public:
     };
 
     // detect and initialise any available proximity sensors
-    void init();
+    __INITFUNC__ void init();
 
     // update state of all proximity sensors. Should be called at high rate from main loop
     void update();