net/memif: fix build with gcc 9.1

net/memif: fix build with gcc 9.1

gcc-9 is stricter on NULL arguments for printf.
Fix the following build error by avoiding NULL argument to printf.

In file included from drivers/net/memif/memif_socket.c:26:
In function 'memif_socket_create',
inlined from 'memif_socket_init' at net/memif/memif_socket.c:965:12:
net/memif/rte_eth_memif.h:35:2: error:
'%s' directive argument is null [-Werror=format-overflow=]
   35 |  rte_log(RTE_LOG_ ## level, memif_logtype, \
      |  ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
   36 |   "%s(): " fmt "\n", __func__, ##args)
      |   ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Fixes: 09c7e63a71f9 ("net/memif: introduce memory interface PMD")

Signed-off-by: Jerin Jacob <jerinj@marvell.com>