My last commit c1137b6db7 "libffi: fix
libffi.pc file" broke the host build as that still placed the header
files to /usr/lib/libffi-3.0.13/include/ and the libffi.pc file pointed
to /usr/include/ . With this patch I took the patch from Debian and
also made the install process to put the header files to /usr/include
in all situations.

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>

The include dir pointed to staging_dir/target-mips_34kc+dsp_musl-1.1.11
/usr/lib/libffi-3.0.13/include which does not exists, this made glib2
fail to compile. The header files are copied to /usr/include so define
it in the libffi.pc file.

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>