Add toolchain/* to root.bp

Soong will now scan toolchain/benchmark for any blueprint files.

Test: Cherry-pick
https://android-review.googlesource.com/#/c/toolchain/benchmark/+/474805/,
that has blueprint files in toolchain/benchmark and build.

Change-Id: Ia4ef262094d60423f80f195ea0b95392b740f476
diff --git a/root.bp b/root.bp
index d91da64..f27d28b 100644
--- a/root.bp
+++ b/root.bp
@@ -37,5 +37,6 @@
     "test/vts",
     "test/vts-testcase/*",
     "tools/*",
+    "toolchain/*",
     "vendor/*/*",
 ]