Assume <utmpx.h> always exists

We already made that assumption in commit b47aa1e9aa.  While the
header is not required by POSIX (it is an XSI extension), it is defined
in systems that are of interest to this project (GNU/Linux).

Fixes: b47aa1e9aa ("Assume <utmpx.h> exists")
Cc: Serge Hallyn <serge@hallyn.com>
Signed-off-by: Alejandro Colomar <alx@kernel.org>

configure.ac

@@ -698,9 +698,6 @@ if test "$with_skey" = "yes"; then
 fi
 
 if test "$enable_utmpx" = "yes"; then
-	if test "$ac_cv_header_utmpx_h" != "yes"; then
-		AC_MSG_ERROR([The utmpx.h header file is required for utmpx support.])
-	fi
 	AC_DEFINE(USE_UTMPX,
 	          1,
 	          [Define if utmpx should be used])