schema/defs-linux: Fix type for seccomp names

The:

  "type": [
    "string"
  ]

syntax added in 652323c (improve seccomp format to be more
expressive, 2017-01-13, opencontainers#657) is not valid:

  $ ./validate ./config-schema.json <../config.json
  The document is not valid. see errors :
  - linux.seccomp.syscalls.0.names: Invalid type. Expected: string, given: array

Signed-off-by: W. Trevor King <wking@tremily.us>

schema/defs-linux.json

@@ -66,9 +66,10 @@
             "type": "object",
             "properties": {
                 "names": {
-                    "type": [
-                        "string"
-                    ]
+                    "type": "array",
+                    "items": {
+                        "type": "string"
+                    }
                 },
                 "action": {
                     "$ref": "#/definitions/SeccompAction"