net: sctp: trivial: add uapi/linux/sctp.h into maintainers

After this file has moved to the uapi section, we also need to update
this in the maintainers file.

Signed-off-by: Daniel Borkmann <[e-mail]>
Acked-by: Neil Horman <[e-mail]>
Acked-by: Vlad Yasevich <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

MAINTAINERS

···
 S:	Maintained
 F:	Documentation/networking/sctp.txt
 F:	include/linux/sctp.h
+F:	include/uapi/linux/sctp.h
 F:	include/net/sctp/
 F:	net/sctp/