.gitignore: move *.lex.c *.tab.[ch] patterns to the top-level .gitignore

These patterns are common to host programs that require lexer and parser.
Move them to the top .gitignore.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Acked-by: Frank Rowand <frowand.list@gmail.com>