libffi: add patch to fix library installation location

The libffi library was installed in a wrong directory when using the
non-default variant of a multilib toolchain. We fix this by patching
the libffi Makefile.am to install its library in a normal
location. The patch has been submitted upstream.

Fixes:

 http://autobuild.buildroot.org/results/471b1d1547c8d726999ba9a865b87fd75429dcab/build-end.log (dbus-glib)
 http://autobuild.buildroot.org/results/eda3ae14eea61e777511fca561f8d43dc7f4a2a5/build-end.log (shared-mime-info)
 http://autobuild.buildroot.org/results/9c86692ae3485d5ae371c695bfbb4a9b67aa2368/build-end.log (librsvg)
 http://autobuild.buildroot.org/results/affd7f20c95f1019d040e2911877694300e3c876/build-end.log

 (bustle)

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>