Add missing value_category dep to syntax (#1151)

executable_semantics/syntax/BUILD

@@ -66,6 +66,7 @@ cc_library(
         "//executable_semantics/ast:expression",
         "//executable_semantics/ast:paren_contents",
         "//executable_semantics/ast:pattern",
+        "//executable_semantics/ast:value_category",
         "//executable_semantics/common:arena",
         "//executable_semantics/common:error",
         "//executable_semantics/common:nonnull",