unlinkat.2: PROTOTYPE: Add <fcntl.h> for AT_* constants

Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>

man2/unlinkat.2

@@ -28,6 +28,7 @@
 unlinkat \- remove a directory entry relative to a directory file descriptor
 .SH SYNOPSIS
 .nf
+.B #include <fcntl.h>           /* Definition of AT_* constants */
 .B #include <unistd.h>
 .sp
 .BI "int unlinkat(int " dirfd ", const char *" pathname ", int " flags );