cppcheck: Broaden suppression for unused struct members

In a number of places in passt we use structures to represent over the wire
or in-file data with a fixed layout.  After initialization we don't access
the fields individually and just write the structure as a whole to its
destination.

Unfortunately cppcheck doesn't cope with this pattern and thinks all the
structure members are unused.  We already have suppressions for this in
pcap.c and dhcp.c   However, it also appears in dhcp.c and netlink.c at
least.  Since this is likely to be common, it seems wiser to just suppress
the error globally.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Stefano Brivio <sbrivio@redhat.com>

Makefile

@@ -286,10 +286,9 @@ cppcheck: $(SRCS) $(HEADERS)
 	--suppress=objectIndex:tcp.c --suppress=objectIndex:udp.c	\
 	--suppress=va_list_usedBeforeStarted:util.c			\
 	--suppress=unusedFunction					\
+	--suppress=unusedStructMember					\
 	--suppress=knownConditionTrueFalse:conf.c			\
 	--suppress=localtimeCalled:pcap.c				\
-	--suppress=unusedStructMember:pcap.c				\
-	--suppress=unusedStructMember:dhcp.c				\
 									\
 	--suppress=unmatchedSuppression:conf.c				\
 	--suppress=unmatchedSuppression:dhcp.c				\