pmix2x: configury: add missing check for <netdb.h> header file

(back-ported from upstream pmix/master@e54ce6d423)

opal/mca/pmix/pmix2x/pmix/config/pmix.m4

@@ -18,7 +18,7 @@ dnl                         reserved.
 dnl Copyright (c) 2009-2011 Oak Ridge National Labs.  All rights reserved.
 dnl Copyright (c) 2011-2013 NVIDIA Corporation.  All rights reserved.
 dnl Copyright (c) 2013-2015 Intel, Inc. All rights reserved
-dnl Copyright (c) 2015      Research Organization for Information Science
+dnl Copyright (c) 2015-2016 Research Organization for Information Science
 dnl                         and Technology (RIST). All rights reserved.
 dnl Copyright (c) 2016      Mellanox Technologies, Inc.
 dnl                         All rights reserved.
@@ -301,7 +301,8 @@ AC_DEFUN([PMIX_SETUP_CORE],[
                       time.h unistd.h dirent.h \
                       crt_externs.h signal.h \
                       ioLib.h sockLib.h hostLib.h limits.h \
-                      sys/statfs.h sys/statvfs.h])
+                      sys/statfs.h sys/statvfs.h \
+                      netdb.h])
 
     # Note that sometimes we have <stdbool.h>, but it doesn't work (e.g.,
     # have both Portland and GNU installed; using pgcc will find GNU's