make: fix memory leak

`info->symlink_target` is a `std::string`. This `strdup` therefore leaks
memory. Just use `std::string::operator=(const char *)` instead.

Bug: 206470603
Test: TreeHugger
Change-Id: I71bcaec583c7d429b4d09522cc5eeb2ced009007
diff --git a/tools/build-runfiles.cc b/tools/build-runfiles.cc
index d92e663..b6197f0 100644
--- a/tools/build-runfiles.cc
+++ b/tools/build-runfiles.cc
@@ -147,7 +147,7 @@
         info->type = FILE_TYPE_REGULAR;
       } else {
         info->type = FILE_TYPE_SYMLINK;
-        info->symlink_target = strdup(target);
+        info->symlink_target = target;
       }
 
       FileInfo parent_info;