Fixed src directory for protobuf implementation

qgroundcontrol.pro

@@ -550,7 +550,7 @@ contains(DEPENDENCIES_PRESENT, protobuf):contains(MAVLINK_CONF, pixhawk) {
     message("Including sources for Protocol Buffers")
 
     # Enable only if protobuf is available
-    SOURCES += mavlink/src/v1.0/pixhawk/pixhawk.pb.cc \
+    SOURCES += mavlink/share/mavlink/src/v1.0/pixhawk/pixhawk.pb.cc \
         src/ui/map3D/ObstacleGroupNode.cc \
         src/ui/map3D/GLOverlayGeode.cc
 }