[I/OAT]: net/core/user_dma.c should #include <net/netdma.h>

Every file should #include the headers containing the prototypes for
its global functions.

Especially in cases like this one where gcc can tell us through a
compile error that the prototype was wrong...

Signed-off-by: Adrian Bunk <bunk@stusta.de>
Signed-off-by: David S. Miller <davem@davemloft.net>