multimedia/mythtv: unbreak with clang 9

ld: error: ../libmythfreesurround/libmythfreesurround-30.a(el_processor.o): unable to find library from dependent library specifier: libfftw3f-3.lib

Reported by: antoine (via bug 240629 exp-run)
Regressed by: https://reviews.llvm.org/rL360984