Avoids picking up non-existent FFI headers. The logic for their
inclusion into bindist should be conditional, but we don't care about
fixing this better. In ghc 9.4 bindist is bulit via Hadrian and so
this patch becomes irrelevant.

Index: ghc.mk
--- ghc.mk.orig
+++ ghc.mk
@@ -1049,7 +1049,6 @@ $(eval $(call bindist-list,.,\
     $(includes_1_H_VERSION) \
     $(includes_DERIVEDCONSTANTS) \
     $(includes_GHCCONSTANTS) \
-    $(libffi_HEADERS) \
     $(INSTALL_LIBEXECS) \
     $(INSTALL_LIBEXEC_SCRIPTS) \
     $(INSTALL_BINS) \