fork.2, getsid.2, setpgid.2, setsid.2: Include <sys/types.h> in SYNOPSIS for pid_t

vfork(2), getpid(2) and others which return pid_t already do this.

mtk: Additional info from Ahmad: <unistd.h> defines 'pid_t',
but only dependent on certain FTMs beng defined.

Cc: linux-man@vger.kernel.org
Signed-off-by: Ahmad Fatoum <ahmad@a3f.at>
Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>

man2/fork.2

@@ -40,6 +40,8 @@
 .SH NAME
 fork \- create a child process
 .SH SYNOPSIS
+.B #include <sys/types.h>
+.br
 .B #include <unistd.h>
 .PP
 .B pid_t fork(void);

man2/getsid.2

@@ -28,6 +28,8 @@
 .SH NAME
 getsid \- get session ID
 .SH SYNOPSIS
+.B #include <sys/types.h>
+.br
 .B #include <unistd.h>
 .PP
 .BI "pid_t getsid(pid_t" " pid" );

man2/setpgid.2

@@ -49,6 +49,8 @@
 .SH NAME
 setpgid, getpgid, setpgrp, getpgrp \- set/get process group
 .SH SYNOPSIS
+.B #include <sys/types.h>
+.br
 .B #include <unistd.h>
 .PP
 .BI "int setpgid(pid_t " pid ", pid_t " pgid );

man2/setsid.2

@@ -33,6 +33,8 @@
 setsid \- creates a session and sets the process group ID
 .SH SYNOPSIS
 .ad l
+.B #include <sys/types.h>
+.br
 .B #include <unistd.h>
 .PP
 .B pid_t setsid(void);