include/linux/printk.h: remove double asmlinkage in printk_emit

The double asmlinkage was introduced in commit 7ff9554b ("printk:
convert byte-buffer to variable-length record buffer").

Signed-off-by: Simon Kagstrom <simon.kagstrom@netinsight.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>