Fix GCC compilation error

Without the added header, GCC will produce the following error:

error: no member named 'unique_ptr' in namespace 'std'

Change-Id: I1d4055213b63163431bc0fd7304d48355bca8770
Reviewed-on: https://gn-review.googlesource.com/c/gn/+/8280
Reviewed-by: Brett Wilson <brettw@chromium.org>
Commit-Queue: Brett Wilson <brettw@chromium.org>

src/gn/string_output_buffer.h

diff --git a/src/gn/string_output_buffer.h b/src/gn/string_output_buffer.h
index b42b638..d2a42c9 100644
--- a/src/gn/string_output_buffer.h
+++ b/src/gn/string_output_buffer.h
@@ -6,6 +6,7 @@
 #define TOOLS_GN_STRING_OUTPUT_BUFFER_H_
 
 #include <array>
+#include <memory>
 #include <streambuf>
 #include <string>
 #include <vector>