FIRDesigner multiplies is deprecated

[guruofquality]

Looked like an ominous warning

filter/FIRDesigner.cpp:462:25: warning: 'bind1st<std::__1::multiplies<std::__1::complex<double>>, double>' is deprecated [-Wdeprecated-declarations]
                   std::bind1st(std::multiplies<std::complex<double>>(),_gain));

[ncorgan]

Lambdas are nicer anyway.