.gitignore: drop include/asm/proc from ignore pattern

Commit 7d89982b stopped creating symbolic link
arch/${arch}/include/asm/proc.

arch/.gitignore should be updated.

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
Cc: Vasili Galka <vvv444@gmail.com>