configure: remove duplicit check for sys/un.h

Commit 7c08fcc4 added this one.
Signed-off-by: NJán Tomko <jtomko@redhat.com>
Reviewed-by: NErik Skultety <eskultet@redhat.com>

configure.ac

@@ -369,7 +369,6 @@ AC_CHECK_HEADERS([\
   sys/sysctl.h \
   sys/ucred.h \
   sys/un.h \
-  sys/un.h \
   ])
 dnl Check whether endian provides handy macros.
 AC_CHECK_DECLS([htole64], [], [], [[#include <endian.h>]])