genksyms: drop *.hash.c from .gitignore

This is a left-over of commit bb3290d9 ("Remove gperf usage from
toolchain").

We do not generate a hash function any more.
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>

scripts/genksyms/.gitignore

-*.hash.c
 *.lex.c
 *.tab.c
 *.tab.h