[build] Specify source_root

See more details about source_root in https://fxrev.dev/515901.

Change-Id: I56ebb25f84a0c1595f548f34548b66b207ecd7c5
Reviewed-on: https://fuchsia-review.googlesource.com/c/third_party/pyyaml/+/515687
Reviewed-by: Shai Barack <shayba@google.com>
diff --git a/BUILD.gn b/BUILD.gn
index 14b1d32..c7e5ef6 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -5,23 +5,24 @@
 import("//build/python/python_library.gni")
 
 python_library("yaml") {
+  source_root = "lib3/yaml"
   sources = [
-    "lib3/yaml/__init__.py",
-    "lib3/yaml/composer.py",
-    "lib3/yaml/constructor.py",
-    "lib3/yaml/cyaml.py",
-    "lib3/yaml/dumper.py",
-    "lib3/yaml/emitter.py",
-    "lib3/yaml/error.py",
-    "lib3/yaml/events.py",
-    "lib3/yaml/loader.py",
-    "lib3/yaml/nodes.py",
-    "lib3/yaml/parser.py",
-    "lib3/yaml/reader.py",
-    "lib3/yaml/representer.py",
-    "lib3/yaml/resolver.py",
-    "lib3/yaml/scanner.py",
-    "lib3/yaml/serializer.py",
-    "lib3/yaml/tokens.py",
+    "__init__.py",
+    "composer.py",
+    "constructor.py",
+    "cyaml.py",
+    "dumper.py",
+    "emitter.py",
+    "error.py",
+    "events.py",
+    "loader.py",
+    "nodes.py",
+    "parser.py",
+    "reader.py",
+    "representer.py",
+    "resolver.py",
+    "scanner.py",
+    "serializer.py",
+    "tokens.py",
   ]
 }