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: mfbt/Casting.h
--- mfbt/Casting.h.orig
+++ mfbt/Casting.h
@@ -12,7 +12,7 @@
 #include "mozilla/Assertions.h"
 #include "mozilla/Sprintf.h"
 
-#include "fmt/format.h"
+#include "../fmt/format.h"
 
 #include <cinttypes>
 #include <cstring>
