Remove need for and use of ioprio header

src/ioprio.cpp

@@ -1,7 +1,6 @@
 #include "ioprio.hpp"
 
 #ifdef __linux__
-# include <linux/ioprio.h>
 # include <sys/syscall.h>
 # include <unistd.h>
 # include <errno.h>
@@ -9,6 +8,11 @@
 #warning "ioprio not supported on this platform"
 #endif
 
+enum
+  {
+    IOPRIO_WHO_PROCESS = 1
+  };
+
 thread_local int ioprio::SetFrom::thread_prio = -1;
 bool _enabled = false;