scripts/genksyms: clean lex/yacc generated files

Add "keywords.hash.c", "lex.lex.c", "parse.tab.c" and "parse.tab.h" to
clean-list so that they get automagically deleted at clean/mrproper
time.

Signed-off-by: Fernando Luis Vazquez <Cao&lt;fernando@oss.ntt.co.jp>
Signed-off-by: Michal Marek <mmarek@suse.cz>