sd-json: properly export sd_json_variant_type_from_string() and _to_string()

These exist in the header file, but were not exporeted.

Continuation of e11f5aa722.

src/libsystemd/libsystemd.sym

@@ -1062,5 +1062,7 @@ global:
 
 LIBSYSTEMD_258 {
 global:
+        sd_json_variant_type_from_string;
+        sd_json_variant_type_to_string;
         sd_varlink_reset_fds;
 } LIBSYSTEMD_257;

src/libsystemd/sd-json/sd-json.c

@@ -5808,4 +5808,4 @@ static const char* const sd_json_variant_type_table[_SD_JSON_VARIANT_TYPE_MAX] =
         [SD_JSON_VARIANT_NULL]     = "null",
 };
 
-DEFINE_STRING_TABLE_LOOKUP(sd_json_variant_type, sd_json_variant_type_t);
+_DEFINE_STRING_TABLE_LOOKUP(sd_json_variant_type, sd_json_variant_type_t, _public_);