Disable -Waddress-of-packed-member on Linux

Linux uses packed structures for sctp_paddrinfo, sctp_paddrparams,
sctp_prim, and sctp_setpeerprim resulting in not being able to
access some fields directly. Not sure why this is done the way
it is on Linux...

gtests/net/packetdrill/Makefile.Linux

 packetdrill-ext-libs := -lpthread -lrt -ldl -lsctp -static
-CFLAGS = -Wall -Werror
+CFLAGS = -Wall -Werror -Wno-address-of-packed-member
 include Makefile.common