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Add `libfl-dev` to the dependencies on debian to prevent the following error:
```
[ 16%] Building CXX object src/cc/frontends/b/CMakeFiles/b_frontend.dir/loader.cc.o
In file included from /root/bcc/src/cc/frontends/b/parser.h:21:0,
                 from /root/bcc/src/cc/frontends/b/loader.cc:17:
/root/bcc/src/cc/frontends/b/lexer.h:22:23: fatal error: FlexLexer.h: No such file or directory
 #include <FlexLexer.h>
                       ^
compilation terminated.
```