Mark unused parameters and set -Wunused-parameter

This allows us to compile with -Wall -Werror in build
environments where -Wall also warns about unused parameters.
Also explicitly set -Wunused-parameter so unused parameters will
cause the build to fail in environments where -Wall does not warn
about them.

Change-Id: Icccf2121d2a9df77b1c224c4976cb9aec56496b3
diff --git a/getaddr.c b/getaddr.c
index fb761f0..5cae78b 100644
--- a/getaddr.c
+++ b/getaddr.c
@@ -85,7 +85,9 @@
  * err  - netlink message
  * arg  - (struct target) info for which address we're looking for
  */
-static int error_handler(struct sockaddr_nl *nla, struct nlmsgerr *err, void *arg) {
+static int error_handler(__attribute__((unused)) struct sockaddr_nl *nla,
+                         __attribute__((unused)) struct nlmsgerr *err,
+                         __attribute__((unused)) void *arg) {
   return NL_OK;
 }