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