Prevent the vendored fmt headers from being shadowed by devel/fmt.

Consumers pass -isystem ${LOCALBASE}/include/mozjs-140, which is
searched after every -I directory, so a bare "fmt/format.h" resolves
to ${LOCALBASE}/include/fmt/format.h whenever devel/fmt happens to be
installed. Use a path relative to this header so the copy shipped with
mozjs is always picked; the quoted include still falls back to the
regular search path within the mozjs build tree.

Index: js/public/experimental/LoggingInterface.h
--- js/public/experimental/LoggingInterface.h.orig
+++ js/public/experimental/LoggingInterface.h
@@ -13,7 +13,7 @@
 #include "mozilla/LoggingCore.h"
 
 #include "jstypes.h"
-#include "fmt/format.h"
+#include "../../fmt/format.h"
 #include "js/GCAPI.h"
 
 struct JSContext;
