imagemagick: add fftw to DEPENDS

* it's autodetected and then later when it's not in sysroot it fails:
| /bin/grep: /OE/shr-core/tmp-eglibc/sysroots/tuna/usr/lib/libfftw3.la: No such file or directory
| /bin/sed: can't read /OE/shr-core/tmp-eglibc/sysroots/tuna/usr/lib/libfftw3.la: No such file or directory
| arm-oe-linux-gnueabi-libtool: link: `/OE/shr-core/tmp-eglibc/sysroots/tuna/usr/lib/libfftw3.la' is not a valid libtool archive
| make[1]: *** [wand/libMagickWand.la] Error 1
| make[1]: Leaving directory `/OE/shr-core/tmp-eglibc/work/armv7a-vfp-neon-oe-linux-gnueabi/imagemagick-6.7.5-r3/ImageMagick-6.7.5-6'

Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>