Export rustflags in the IDE JSON format

These are currently omitted but should be included similarly to other
compiler flags.

Change-Id: I3753dcd966f8f0db6ed1b37374cbeb739764de8a
Reviewed-on: https://gn-review.googlesource.com/c/gn/+/6801
Reviewed-by: Brett Wilson <brettw@chromium.org>
Commit-Queue: Petr Hosek <phosek@google.com>

src/gn/desc_builder.cc

diff --git a/src/gn/desc_builder.cc b/src/gn/desc_builder.cc
index 57b253d..8e84d38 100644
--- a/src/gn/desc_builder.cc
+++ b/src/gn/desc_builder.cc
@@ -463,6 +463,7 @@
       CONFIG_VALUE_ARRAY_HANDLER(cflags_cc, std::string)
       CONFIG_VALUE_ARRAY_HANDLER(cflags_objc, std::string)
       CONFIG_VALUE_ARRAY_HANDLER(cflags_objcc, std::string)
+      CONFIG_VALUE_ARRAY_HANDLER(rustflags, std::string)
       CONFIG_VALUE_ARRAY_HANDLER(defines, std::string)
       CONFIG_VALUE_ARRAY_HANDLER(include_dirs, SourceDir)
       CONFIG_VALUE_ARRAY_HANDLER(inputs, SourceFile)