[fusl] Fix typo in action output

This was causing fusl builds to always be dirty.

Fixes #657

R=viettrungluu@chromium.org

Review URL: https://codereview.chromium.org/1681933004 .
diff --git a/fusl/crt/BUILD.gn b/fusl/crt/BUILD.gn
index ef90f56..1a6ab21 100644
--- a/fusl/crt/BUILD.gn
+++ b/fusl/crt/BUILD.gn
@@ -136,7 +136,7 @@
 
   outputs = [
     "$target/crtbegin.o",
-    "$target/crtbeginSo.o",
+    "$target/crtbeginS.o",
     "$target/crtbeginT.o",
     "$target/crtend.o",
     "$target/crtendS.o",