Include net/if.h before net/if_dl.h

In old versions of macOS, net/if_dl.h neglects to include sys/types.h,
which results in build failure:

error: unknown type name 'u_char'; did you mean 'char'?

Including net/if.h first works around the problem because net/if.h
includes sys/types.h.

https://github.com/giampaolo/psutil/issues/2360
https://github.com/giampaolo/psutil/commit/494d8b84f601a19f4f298ffbb3b47a647384d521
--- psutil/arch/osx/net.c.orig
+++ psutil/arch/osx/net.c
@@ -9,11 +9,11 @@
 // https://github.com/giampaolo/psutil/blame/efd7ed3/psutil/_psutil_osx.c
 
 #include <Python.h>
+#include <net/if.h>
 #include <net/if_dl.h>
 #include <net/route.h>
 #include <sys/sysctl.h>
 #include <sys/socket.h>
-#include <net/if.h>
 
 #include "../../_psutil_common.h"