HAL_PX4: cope with PX4 header changes

libraries/AP_HAL_PX4/RCInput.h

@@ -7,6 +7,11 @@
 #include <systemlib/perf_counter.h>
 #include <pthread.h>
 
+
+#ifndef RC_INPUT_MAX_CHANNELS
+#define RC_INPUT_MAX_CHANNELS 18
+#endif
+
 class PX4::PX4RCInput : public AP_HAL::RCInput {
 public:
     void init(void* machtnichts);