Another OpenGL library build system fix (#2842)

sky/shell/platform/mojo/BUILD.gn

diff --git a/sky/shell/platform/mojo/BUILD.gn b/sky/shell/platform/mojo/BUILD.gn
index 2204008..5b228a7 100644
--- a/sky/shell/platform/mojo/BUILD.gn
+++ b/sky/shell/platform/mojo/BUILD.gn
@@ -48,7 +48,9 @@
     "//third_party/icu",
   ]
 
-  ldflags = [
-    "-lGL",
-  ]
+  if (is_linux) {
+    ldflags = [
+      "-lGL",
+    ]
+  }
 }