Add `#include <memory>` in src/gn/err.h

Without this ninja fails with:

../src/gn/err.h:113:8: error: no template named 'unique_ptr' in namespace 'std'
  std::unique_ptr<ErrInfo> info_;  // Non-null indicates error.

Change-Id: I42e4b72f3c14c20cc4a0d2d9a8c74647f6449706
Reviewed-on: https://gn-review.googlesource.com/c/gn/+/12760
Reviewed-by: Brett Wilson <brettw@chromium.org>
Commit-Queue: Brett Wilson <brettw@chromium.org>

src/gn/err.h

diff --git a/src/gn/err.h b/src/gn/err.h
index efccf0b..662d8b5 100644
--- a/src/gn/err.h
+++ b/src/gn/err.h
@@ -5,6 +5,7 @@
 #ifndef TOOLS_GN_ERR_H_
 #define TOOLS_GN_ERR_H_
 
+#include <memory>
 #include <string>
 #include <vector>