.gitignore: drop include/asm from ignored file list

Commit bb02c536 stopped creaing a symbolic link include/asm.

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>

include/.gitignore

 /autoconf.mk*
-/asm
 /bmp_logo.h
 /bmp_logo_data.h
 /config.h