libmemif: ubuntu 18.04 build fix

Change-Id: I6624cacd625eea6a9b8214f67d14ec162e2c0195
Signed-off-by: Jakub Grajciar <jgrajcia@cisco.com>
diff --git a/extras/libmemif/configure.ac b/extras/libmemif/configure.ac
index 323e113..dfe33cf 100644
--- a/extras/libmemif/configure.ac
+++ b/extras/libmemif/configure.ac
@@ -7,6 +7,8 @@
 PKG_CHECK_MODULES([CHECK], [check], [HAVE_CHECK=1], [HAVE_CHECK=0])
 AM_CONDITIONAL([USE_CHECK],[test "$HAVE_CHECK" -eq 1])
 
+AC_CHECK_FUNC([memfd_create], [AC_DEFINE([HAVE_MEMFD_CREATE], [1], [Define if memfd exists])])
+
 AC_PROG_CC
 
 AC_OUTPUT([Makefile])