Fix build errors on Windows GN

DXGI was changed to load dynamically. These changes need
to be propagated to GN.
diff --git a/BUILD.gn b/BUILD.gn
index 3449410..261389d 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -97,6 +97,8 @@
       sources += [
         "loader/dirent_on_windows.c",
         "loader/dirent_on_windows.h",
+        "loader/dxgi_loader.c",
+        "loader/dxgi_loader.h",
       ]
       if (!is_clang) {
         cflags = [
@@ -112,6 +114,9 @@
           "/wd4996",  # Unsafe stdlib function
         ]
       }
+      if (is_clang) {
+          cflags = [ "-Wno-incompatible-pointer-types" ]
+      }
     }
     if (is_mac) {
       libs = [ "CoreFoundation.framework" ]