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.

tests/test_rand.c

@@ -23,13 +23,25 @@
 /* stubs to build test without whole libstrophe */
 void *xmpp_alloc(const xmpp_ctx_t *const ctx, const size_t size)
 {
+    (void)ctx;
+    (void)size;
     return NULL;
 }
-void xmpp_free(const xmpp_ctx_t *const ctx, void *p) {}
+
+void xmpp_free(const xmpp_ctx_t *const ctx, void *p)
+{
+    (void)ctx;
+    (void)p;
+}
+
 int xmpp_snprintf(char *str, size_t count, const char *fmt, ...)
 {
+    (void)str;
+    (void)count;
+    (void)fmt;
     return 0;
 }
+
 uint64_t time_stamp(void)
 {
     return 0;