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/capability.h b/include/linux/capability.h
index 5f3c63d..dbc21c7 100644
--- a/include/linux/capability.h
+++ b/include/linux/capability.h
@@ -38,6 +38,7 @@
 struct file;
 struct inode;
 struct dentry;
+struct task_struct;
 struct user_namespace;
 
 extern const kernel_cap_t __cap_empty_set;