LibC: Remove duplicate definition of sched_param from sched.h

We were lucky until now that nobody included both headers that have a
definition for this (identical) struct.

Userland/Libraries/LibC/sched.h

@@ -6,6 +6,7 @@
 
 #pragma once
 
+#include <Kernel/API/POSIX/sched.h>
 #include <sys/cdefs.h>
 #include <sys/types.h>
 
@@ -13,10 +14,6 @@ __BEGIN_DECLS
 
 int sched_yield(void);
 
-struct sched_param {
-    int sched_priority;
-};
-
 #define SCHED_FIFO 0
 #define SCHED_RR 1
 #define SCHED_OTHER 2