async: execution: suppress clang warning of GCC diagnostic

clang doesn't recognize the diagnostic "-Wnon-template-friend" so
suppress it only on GCC.

Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
Change-Id: I4510a46bb4994a5593c458b34cffbebce1128651
diff --git a/include/sdbusplus/async/execution.hpp b/include/sdbusplus/async/execution.hpp
index a7f02e7..1de17e0 100644
--- a/include/sdbusplus/async/execution.hpp
+++ b/include/sdbusplus/async/execution.hpp
@@ -4,7 +4,9 @@
 // as needed.
 
 #pragma GCC diagnostic push
+#ifndef __clang__
 #pragma GCC diagnostic ignored "-Wnon-template-friend"
+#endif
 #pragma GCC diagnostic ignored "-Wmissing-field-initializers"
 #include <sdbusplus/async/stdexec/coroutine.hpp>
 #include <sdbusplus/async/stdexec/execution.hpp>