authd/providers/opm.c: include <netinet/tcp.h> (#318)

This header defines the TCP_NODELAY flag, which this compilation
unit uses.

Other C libraries implicitly include this header from some other
header we are using (I have not investigated which), but musl's
system headers do not, which breaks building on musl.

Reported-by: 0x5c <dev@0x5c.io>

authd/providers/opm.c

@@ -26,6 +26,8 @@
 #include "notice.h"
 #include "provider.h"
 
+#include <netinet/tcp.h> // TCP_NODELAY
+
 #define SELF_PID (opm_provider.id)
 
 #define OPM_READSIZE 128