tools/testing/selftests/proc/proc-self-syscall.c: remove duplicate include

Remove duplicate header which is included twice.

Link: http://lkml.kernel.org/r/20190304182719.GA6606@jordon-HP-15-Notebook-PC


Signed-off-by: Sabyasachi Gupta <sabyasachi.linux@gmail.com>
Signed-off-by: Souptick Joarder <jrdr.linux@gmail.com>
Reviewed-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>