Mark unused functions for cppcheck

We have a couple of functions that are unused (for now) by design.
Although at least one has a flag so that gcc doesn't warn, cppcheck has its
own warnings about this.  Add specific inline suppressions for these rather
than a blanket suppression in the Makefile.

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

Makefile

@@ -283,7 +283,6 @@ cppcheck: $(SRCS) $(HEADERS)
 	$(SYSTEM_INCLUDES:%=--suppress=*:%/*)				\
 	$(SYSTEM_INCLUDES:%=--suppress=unmatchedSuppression:%/*)	\
 	--inline-suppr							\
-	--suppress=unusedFunction					\
 	--suppress=unusedStructMember					\
 									\
 	--suppress=unmatchedSuppression:dhcp.c				\

igmp.c

@@ -13,4 +13,5 @@
  */
 
 /* TO BE IMPLEMENTED */
+/* cppcheck-suppress unusedFunction */
 __attribute__((__unused__)) static void unused(void) { }

siphash.c

@@ -177,6 +177,7 @@ uint64_t siphash_20b(const uint8_t *in, const uint64_t *k)
  *
  * Return: the 64-bit hash output
  */
+/* cppcheck-suppress unusedFunction */
 uint32_t siphash_32b(const uint8_t *in, const uint64_t *k)
 {
 	uint64_t *in64 = (uint64_t *)in;