Add missing syntax deps (#1014)

executable_semantics/syntax/BUILD

@@ -62,8 +62,11 @@ cc_library(
         "//executable_semantics/ast:declaration",
         "//executable_semantics/ast:expression",
         "//executable_semantics/ast:paren_contents",
+        "//executable_semantics/ast:pattern",
         "//executable_semantics/common:arena",
         "//executable_semantics/common:error",
+        "//executable_semantics/common:nonnull",
+        "@llvm-project//llvm:Support",
     ],
 )