arch/ia64/kernel/head.S: remove duplicate include

'linux/pgtable.h' included in 'arch/ia64/kernel/head.S' is duplicated.

Link: https://lkml.kernel.org/r/20210303084549.179346-1-zhang.yunkai@zte.com.cn


Signed-off-by: Zhang Yunkai <zhang.yunkai@zte.com.cn>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>