bc: depend on flex for Linuxbrew
bc: depend on flex for Linuxbrew

on Linux, `bc` binary uses `libfl.so.2` which is provided by `flex`.

Closes Linuxbrew/homebrew-core#2345.

Signed-off-by: Maxim Belkin <032baa6ccf1d4eaa4536fc4f9b83b9a1f593449a@gmail.com>

Linuxbrew/homebrew-core,LinuxbrewTestBot/homebrew-core,LinuxbrewTestBot/homebrew-core,maxim-belkin/homebrew-core,Linuxbrew/homebrew-core,rwhogg/homebrew-core,maxim-belkin/homebrew-core,rwhogg/homebrew-core