fix use of memset without declaration in sched.h cpu set macros

patch by Jörg Krause.

include/sched.h

@@ -72,6 +72,7 @@ int setns(int, int);
 
 void *memcpy(void *__restrict, const void *__restrict, size_t);
 int memcmp(const void *, const void *, size_t);
+void *memset (void *, int, size_t);
 void *calloc(size_t, size_t);
 void free(void *);