AP_RCProtocol: get rid of compiler warning from clang about unknown w…

…arning group -Wswitch-unreachable

libraries/AP_RCProtocol/AP_RCProtocol.h

@@ -128,11 +128,13 @@ class AP_RCProtocol {
         _disabled_for_pulses |= (1U<<(uint8_t)protocol);
     }
 
+#if !defined(__clang__)
 // in the case we've disabled most backends then the "return true" in
 // the following method can never be reached, and the compiler gets
 // annoyed at that.
 #pragma GCC diagnostic push
 #pragma GCC diagnostic ignored "-Wswitch-unreachable"
+#endif
 
     // for protocols without strong CRCs we require 3 good frames to lock on
     bool requires_3_frames(enum rcprotocol_t p) {