MAINTAINERS: add Guvenc as SMC maintainer

Add Guvenc as maintainer for Shared Memory Communications (SMC)
Sockets.

Cc: Julian Wiedmann <jwi@linux.ibm.com>
Acked-by: Guvenc Gulce <guvenc@linux.ibm.com>
Signed-off-by: Karsten Graul <kgraul@linux.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>