docs(INSTALL): add libfl-dev to ubuntu dependencies
docs(INSTALL): add libfl-dev to ubuntu dependencies

To compile the software on Ubuntu 20.04 I had trouble compiling the source because my OS is missing the `FlexLexer.h` library. This package is supplied by `libfl-dev`.
brendangregg/bcc,iovisor/bcc,brendangregg/bcc,iovisor/bcc,iovisor/bcc,iovisor/bcc,brendangregg/bcc,brendangregg/bcc,iovisor/bcc,brendangregg/bcc