fix include for escape.h

This is to fix Windows build error with MSVC's STL.

Change-Id: Ic7d0e96e8c0c9458f94f8830b119da9867c50d62
Reviewed-on: https://gn-review.googlesource.com/c/gn/+/17921
Reviewed-by: Dirk Pranke <dpranke@google.com>
Commit-Queue: Takuto Ikuta <tikuta@google.com>
Reviewed-by: David Turner <digit@google.com>

src/gn/escape.h

diff --git a/src/gn/escape.h b/src/gn/escape.h
index 57a723f..c46d42d 100644
--- a/src/gn/escape.h
+++ b/src/gn/escape.h
@@ -7,6 +7,7 @@
 
 #include <iosfwd>
 #include <string_view>
+#include <string>
 
 enum EscapingMode {
   // No escaping.