fftw: s/PN/BPN/

Replace the PN with BPN to fix the multilib build.

The error is:
/ala-lpggp21/bpiolin/intel-x86-64-gplv2-4/bitbake_build/tmp/pkgdata/*/runtime-reverse/lib32-libfftw:
No such file or directory

Because the PN causes the package is named as lib32-liblib32-fftw.

Signed-off-by: Yue Tao <Yue.Tao@windriver.com>
Signed-off-by: Jackie Huang <jackie.huang@windriver.com>
Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>