only depend on libiberty on linux

Summary: demangle.h isn't available on macOS, so only include it on Linux.

Reviewed By: andrewjcg

Differential Revision: D14114593

fbshipit-source-id: 59b12bfb4a52ada636b3648115dec1383f58c22f

folly/detail/Demangle.cpp

@@ -23,7 +23,7 @@
 // So we extract the inclusion of `<demangle.h>` which includes `<libiberty.h>`
 // to here, isolating it.
 #if FOLLY_DETAIL_HAVE_DEMANGLE_H
-#include <demangle.h>
+#include <demangle.h> // @manual
 #endif
 
 namespace folly {