Remove -Wno-unused-parameter

Introduced UNUSED macro with cast to void in commoh.h for internal
use. Used cast to void directly in those files which do not
include common.h. Although this change doesn't fix semantic issues
with unused function parameters, it does explicitly mark all those
places, which might require attention in future.

src/sasl.c

@@ -184,6 +184,8 @@ static char *_add_key(xmpp_ctx_t *ctx,
     const char *value, *qvalue;
     char *c;
 
+    UNUSED(len);
+
     /* allocate a zero-length string if necessary */
     if (buf == NULL) {
         buf = xmpp_alloc(ctx, 1);
@@ -405,6 +407,8 @@ char *sasl_scram(xmpp_ctx_t *ctx,
     size_t response_len;
     size_t auth_len;
 
+    UNUSED(jid);
+
     tmp = xmpp_strdup(ctx, challenge);
     if (!tmp) {
         return NULL;