don't fatal if libfido2 not found

spotted by dtucker@
diff --git a/configure.ac b/configure.ac
index 6e32374..3814db6 100644
--- a/configure.ac
+++ b/configure.ac
@@ -3088,8 +3088,7 @@
 			AC_DEFINE([ENABLE_SK_INTERNAL], [],
 			    [Enable for built-in U2F/FIDO support])
 			enable_sk="built-in"
-		],
-		[ AC_MSG_ERROR([libfido2 not found]) ],
+		], [ ],
 		[ $OTHERLIBS ]
 	)
 fi