Add support for WGSL reading to BUILD.gn

BUG=tint:47

Change-Id: Ic50421f4dc712c614cf5783027ba273b53b2bd83
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/19681
Reviewed-by: dan sinclair <dsinclair@google.com>
diff --git a/tint_overrides_with_defaults.gni b/tint_overrides_with_defaults.gni
index 7fb33e5..f302f38 100644
--- a/tint_overrides_with_defaults.gni
+++ b/tint_overrides_with_defaults.gni
@@ -46,6 +46,11 @@
     tint_build_spv_writer = false
   }
 
+  # Build the WGSL input reader
+  if (!defined(tint_build_wgsl_reader)) {
+    tint_build_wgsl_reader = false
+  }
+
   # TODO(rharrison): Implement support for the reset of the reader/writers
 
   # Generate fuzzers