Change function return value to void as it does not return anything, removes warning

[ihatetoregister]

Minor fix for the run_for_frequencies function that was declared as int but did not return anything and generated a warning:

$ g++ -o sparsnas_decode -O3 sparsnas_decode.cpp
sparsnas_decode.cpp:310:1: warning: control reaches end of non-void function [-Wreturn-type]
}
^
1 warning generated.