LibC: Add getpriority() and setpriority() stubs

Expected behavior left as a FIXME is outlined here:
https://pubs.opengroup.org/onlinepubs/7908799/xsh/getpriority.html

Userland/Libraries/LibC/CMakeLists.txt

@@ -22,6 +22,7 @@ set(LIBC_SOURCES
     net.cpp
     netdb.cpp
     poll.cpp
+    priority.cpp
     pthread_forward.cpp
     pthread_integration.cpp
     pthread_tls.cpp

Userland/Libraries/LibC/priority.cpp

@@ -0,0 +1,23 @@
+/*
+ * Copyright (c) 2021, the SerenityOS developers.
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include <AK/Format.h>
+#include <sys/resource.h>
+
+extern "C" {
+
+int getpriority([[maybe_unused]] int which, [[maybe_unused]] id_t who)
+{
+    dbgln("FIXME: Implement getpriority()");
+    return -1;
+}
+
+int setpriority([[maybe_unused]] int which, [[maybe_unused]] id_t who, [[maybe_unused]] int value)
+{
+    dbgln("FIXME: Implement setpriority()");
+    return -1;
+}
+}

Userland/Libraries/LibC/sys/resource.h

@@ -58,4 +58,11 @@ struct rlimit {
 int getrlimit(int, struct rlimit*);
 int setrlimit(int, struct rlimit const*);
 
+#define PRIO_PROCESS 0
+#define PRIO_PGRP 1
+#define PRIO_USER 2
+
+int getpriority(int, id_t);
+int setpriority(int, id_t, int);
+
 __END_DECLS