uapi: move forward declarations of internal structures

Don't user forward declarations of internal kernel structures in headers
exported to userspace.

Move "struct completion;".
Move "struct task_struct;".

Link: http://lkml.kernel.org/r/20160713215808.GA22486@p183.telecom.by
Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
diff --git a/include/linux/sysctl.h b/include/linux/sysctl.h
index fa7bc29..697e160 100644
--- a/include/linux/sysctl.h
+++ b/include/linux/sysctl.h
@@ -28,6 +28,7 @@
 #include <uapi/linux/sysctl.h>
 
 /* For the /proc/sys support */
+struct completion;
 struct ctl_table;
 struct nsproxy;
 struct ctl_table_root;