pidfd_open.2: Add <sys/types.h> to SYNOPSIS

Reported-by: Florian Weimer <fw@deneb.enyo.de>
Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>

man2/pidfd_open.2

@@ -27,6 +27,8 @@
 pidfd_open \- obtain a file descriptor that refers to a process
 .SH SYNOPSIS
 .nf
+.B #include <sys/types.h>
+.PP
 .BI "int pidfd_open(pid_t " pid ", unsigned int " flags );
 .fi
 .SH DESCRIPTION
@@ -120,6 +122,7 @@ returns, and indicates that the file descriptor is readable.
 \&
 .nf
 #define _GNU_SOURCE
+#include <sys/types.h>
 #include <sys/syscall.h>
 #include <unistd.h>
 #include <poll.h>