BUILD.gn: use public_deps for the proxy group.

Otherwise the public_configs of the proxied targets don't correctly get
propagated, causing Dawn to fail compilation due to missing include
directories.

Bug: 706
Change-Id: Ib8acfb06105e77ad005c3900975e8173d85dcf32
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/44680
Auto-Submit: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Ben Clayton <bclayton@google.com>
Reviewed-by: Ben Clayton <bclayton@google.com>

BUILD.gn

@@ -18,7 +18,7 @@ import("tint_overrides_with_defaults.gni")
 # Temporary group to not break dependents while they rename their Tint GN
 # targets
 group("libtint") {
-  deps = [ "src:libtint" ]
+  public_deps = [ "src:libtint" ]
 }
 
 # This target is built when no specific target is specified on the command line.