AP_GPS: unify singleton naming to _singleton and get_singleton()

libraries/AP_GPS/AP_GPS.cpp

@@ -561,7 +561,7 @@ AP_GPS::GPS_Status AP_GPS::highest_supported_status(uint8_t instance) const
 
 bool AP_GPS::should_df_log() const
 {
-    AP_Logger *instance = AP_Logger::instance();
+    AP_Logger *instance = AP_Logger::get_singleton();
     if (instance == nullptr) {
         return false;
     }
@@ -1546,7 +1546,7 @@ namespace AP {
 
 AP_GPS &gps()
 {
-    return AP_GPS::gps();
+    return AP_GPS::get_singleton();
 }
 
 };

libraries/AP_GPS/AP_GPS.h

@@ -71,7 +71,7 @@ public:
     AP_GPS(const AP_GPS &other) = delete;
     AP_GPS &operator=(const AP_GPS&) = delete;
 
-    static AP_GPS &gps() {
+    static AP_GPS &get_singleton() {
         return *_singleton;
     }