mk: ignore missing field initializers warning

mk: ignore missing field initializers warning

Three warnings are commonly disabled in DPDK with make and meson:
	* address-of-packed-member
		always disabled
	* missing-field-initializers
		disabled with meson
		disabled with make + clang or make + gcc < 4.7
		disabled with make + gcc <= 5 for test files and event drivers
	* packed-not-aligned
		disabled with meson

This change is removing exceptions for missing-field-initializers.
As it is always disabled, some redundant configs are cleaned up.

Now the situation is:
	* address-of-packed-member
		always disabled
	* missing-field-initializers
		always disabled
	* packed-not-aligned
		disabled with meson

It could alternatively be decided to disable missing-field-initializers
only for old gcc (< 6).

The warning packed-not-aligned is not modified in this change.

Signed-off-by: Thomas Monjalon <thomas@monjalon.net>
Acked-by: Ferruh Yigit <ferruh.yigit@intel.com>