rts/package.conf.in: fix UNREG on --with-system-libffi when include-dir is passed explicitely

Issue #8748

Signed-off-by: Sergei Trofimovich <slyfox@gentoo.org>
Signed-off-by: Austin Seipp <austin@well-typed.com>
(cherry picked from commit 2d0fa9ae)

rts/package.conf.in

@@ -57,9 +57,9 @@ extra-libraries:
 #endif
 
 #ifdef INSTALLING
-include-dirs:           INCLUDE_DIR PAPI_INCLUDE_DIR
+include-dirs:           INCLUDE_DIR PAPI_INCLUDE_DIR FFI_INCLUDE_DIR
 #else /* !INSTALLING */
-include-dirs:           TOP"/rts/dist/build" TOP"/includes" TOP"/includes/dist-derivedconstants/header"
+include-dirs:           TOP"/rts/dist/build" TOP"/includes" TOP"/includes/dist-derivedconstants/header" FFI_INCLUDE_DIR
 #endif
 
 includes:               Stg.h