[Retrace] Add ReMapper as an x20 dependency

Bug: 149974481
Change-Id: I905c52bb663c3710820ce8f8d5c333a79520779f
diff --git a/.gitignore b/.gitignore
index 105ce45..76604f8 100644
--- a/.gitignore
+++ b/.gitignore
@@ -133,6 +133,8 @@
 third_party/r8.tar.gz
 third_party/r8mappings
 third_party/r8mappings.tar.gz
+third_party/remapper
+third_party/remapper.tar.gz
 third_party/retrace_benchmark
 third_party/retrace_benchmark.tar.gz
 third_party/rhino-1.7.10