libffi: remove invalid variable 'includedir' in .pc file

Fixes the following error:

  Variable 'includedir' not defined in '/home/daniel/buildroot/output/host/usr/arm-unknown-linux-gnueabi/sysroot/usr/lib/pkgconfig/libffi.pc'

Also, use $(SED) instead of 'sed -i'.

Signed-off-by: Daniel Mack <zonque@gmail.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>