AC_PID: Add missing const in member functions

Signed-off-by: Patrick José Pereira <patrickelectric@gmail.com>

libraries/AC_PID/AC_PI.h

@@ -18,10 +18,10 @@ public:
     // parameter var table
     static const struct AP_Param::GroupInfo var_info[];
 
-    float get_P() {
+    float get_P() const {
         return output_P;
     }
-    float get_I() {
+    float get_I() const {
         return integrator;
     }