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_ctx.c

@@ -24,18 +24,24 @@ static int mem_realloc_called = 0;
 
 void *my_alloc(const size_t size, void *const userdata)
 {
+    (void)userdata;
+
     mem_alloc_called++;
     return malloc(size);
 }
 
 void my_free(void *p, void *const userdata)
 {
+    (void)userdata;
+
     mem_free_called++;
     return free(p);
 }
 
 void *my_realloc(void *p, const size_t size, void *const userdata)
 {
+    (void)userdata;
+
     mem_realloc_called++;
     return realloc(p, size);
 }
@@ -50,7 +56,7 @@ void my_logger(void *const userdata,
         log_called++;
 }
 
-int main(int argc, char **argv)
+int main()
 {
     xmpp_ctx_t *ctx;
     xmpp_mem_t mymem;