libc tests: dynthr_mod: fix some WARNS issues

This is being addressed as part of a side-patch I'm working on that builds
all the things with WARNS=6, instead of relying on it being supplied in just
shallow parts of the build with higher-level Makefile.inc.

Provide a prototype for mod_main and annotate the thread function argument
as unused.

MFC after:	1 week

lib/libc/tests/stdlib/dynthr_mod/dynthr_mod.c

@@ -41,10 +41,12 @@ __FBSDID("$FreeBSD$");
 #include <unistd.h>
 #include <pthread.h>
 
+void mod_main(int op);
+
 static pthread_t thr;
 
 static void *
-mod_thread(void *ptr)
+mod_thread(void *ptr __unused)
 {
 	char *volatile dummy;