flare-floss: [Enhancement] Don't include library functions, identified by `IDA`, in decoding functions

Currently, floss depends on https://github.com/mandiant/flare-floss/tree/master/sigs to determine the library functions, but in some cases floss includes library functions (identified by IDA FLIRT) in decoding functions. So, can we leverage the IDA FLIRT to exclude them or I think sigs need some update.