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`.

INSTALL.md

@@ -387,7 +387,7 @@ sudo apt-get -y install bison build-essential cmake flex git libedit-dev \
 
 # For Eoan (19.10) or Focal (20.04.1 LTS)
 sudo apt install -y bison build-essential cmake flex git libedit-dev \
- libllvm7 llvm-7-dev libclang-7-dev python zlib1g-dev libelf-dev
+ libllvm7 llvm-7-dev libclang-7-dev python zlib1g-dev libelf-dev libfl-dev
 
 # For other versions
 sudo apt-get -y install bison build-essential cmake flex git libedit-dev \