net/avf: fix missing compiler error flags

The AVF driver was missing $(WERROR_FLAGS) in it's cflags, which means
that a number of compilation errors were getting missed. This patch adds
in the flag and fixes most of the errors, just disabling the
strict-aliasing ones.

Fixes: 22b123a36d07 ("net/avf: initialize PMD")
Fixes: 69dd4c3d0898 ("net/avf: enable queue and device")
Fixes: a2b29a7733ef ("net/avf: enable basic Rx Tx")
Fixes: 319c421f3890 ("net/avf: enable SSE Rx Tx")

CC: stable@dpdk.org
Signed-off-by: Bruce Richardson <bruce.richardson@intel.com>
Reviewed-by: Ferruh Yigit <ferruh.yigit@intel.com>