Files suffixed by .lex.c, .tab.[ch] are generated lexers, parsers,
respectively.  Clean them up globally from the top Makefile.

Some of the final host programs those lexer/parser are linked into
are necessary for building external modules, but the intermediates
are unneeded.  They can be cleaned away by 'make clean' instead of
'make mrproper'.
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Acked-by: Frank Rowand <frowand.list@gmail.com>