net/ionic: ignore missing field initializers warning

The compiler warning is:
   from .../drivers/net/ionic/ionic_dev.c:7:
.../drivers/net/ionic/ionic_if.h:202:5: note: ‘rsvd’ declared here
  u8 rsvd[62];
     ^

This has been observed with gcc 4.8.5, newer 9+ compiler are not giving
this warning.

Warning is a reminder to the user that there are some fields in the
struct not initialized with the default value.
But the C standard clarifies that in that case the field value will be
zero and code is aware of this behavior, so no initializing to a default
value is intentional and it is safe to ignore this compiler warning.

Adding '-Wno-missing-field-initializers' compiler flag to disable the
warning.

Reported-by: Anoob Joseph <anoobj@marvell.com>
Reported-by: Raslan Darawsheh <rasland@mellanox.com>
Reported-by: Xueming Zhang <xuemingx.zhang@intel.com>
Signed-off-by: Ferruh Yigit <ferruh.yigit@intel.com>