add libffi into DEPENDS to fix following build error:
| checking for FFI... no
| configure: error: Package requirements (libffi) were not met:
|
| No package 'libffi' found
(From OE-Core rev: 23d6746efe1b3f31ad156db58fbc2767f750b712)
Signed-off-by: Zhenhua Luo <zhenhua.luo@freescale.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>