sys-auth/sssd: disable realm check for nsupdate

At the moment bind-tools does not enable `gssapi`, so its `nsupdate`
tool is also not able to run `realm` command. As a result, configure
script of `sssd` fails when running `echo realm | nsupdate`, like
`syntax error`.

To avoid such issues, we need to disable the nsupdate check for now.
After we could enable `gssapi` for the SDK correctly, we can bring back
the nsupdate check in the future.

sdk_container/src/third_party/coreos-overlay/sys-auth/sssd/files/sssd-2.3.1-disable-nsupdate-realm.patch

@@ -0,0 +1,10 @@
+--- a/src/external/nsupdate.m4	2020-11-05 16:27:14.661566136 +0100
++++ b/src/external/nsupdate.m4	2020-11-05 16:27:30.060674381 +0100
+@@ -9,7 +9,6 @@
+     AC_MSG_RESULT([yes])
+   else
+     AC_MSG_RESULT([no])
+-    AC_MSG_ERROR([nsupdate does not support 'realm'])
+   fi
+ 
+ else

sdk_container/src/third_party/coreos-overlay/sys-auth/sssd/sssd-2.3.1-r2.ebuild

@@ -113,6 +113,7 @@ MULTILIB_WRAPPED_HEADERS=(
 
 PATCHES=(
 	"${FILESDIR}"/${P}-test_ca-Look-for-libsofthsm2.so-in-usr-libdir-sofths.patch
+	"${FILESDIR}"/${P}-disable-nsupdate-realm.patch
 )
 
 pkg_setup() {