platform incompatible to prevent srcs==None error

srcs cannot be None for a sh_binary, so this target should just be
incompatible with other platforms.

Test: b cquery 'deps(kind(apex, //...))' --config=android
Change-Id: I428fb6003145f10de8dd7904fbf95c511726efd3
diff --git a/BUILD.bazel b/BUILD.bazel
index 92fae3c..eb173f6 100644
--- a/BUILD.bazel
+++ b/BUILD.bazel
@@ -7,6 +7,10 @@
     srcs = select({
         "//build/bazel/platforms/os:darwin": ["darwin-x86/yasm/yasm"],
         "//build/bazel/platforms/os:linux": ["linux-x86/yasm/yasm"],
-        "//conditions:default": None,
+    }),
+    target_compatible_with = select({
+        "//build/bazel/platforms/os:darwin": [],
+        "//build/bazel/platforms/os:linux": [],
+        "//conditions:default": ["@platforms//:incompatible"],
     }),
 )