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: I5ffd7b46d48a90cab00c6fe9b5abac8ce3c84ba1
diff --git a/BUILD.bazel b/BUILD.bazel
index 9e97b25..8b6e198 100644
--- a/BUILD.bazel
+++ b/BUILD.bazel
@@ -44,8 +44,13 @@
         "//build/bazel/platforms/os:darwin": ["darwin-x86/bin/flex"],
         "//build/bazel/platforms/os:linux": ["linux-x86/bin/flex"],
         "//build/bazel/platforms/os_arch:linux_musl_x86": ["linux_musl-x86/bin/flex"],
-        "//conditions:default": None,
-    })
+    }),
+    target_compatible_with = select({
+        "//build/bazel/platforms/os:darwin": [],
+        "//build/bazel/platforms/os:linux": [],
+        "//build/bazel/platforms/os_arch:linux_musl_x86": [],
+        "//conditions:default": ["@platforms//:incompatible"],
+    }),
 )
 
 sh_binary(
@@ -54,7 +59,11 @@
         "//build/bazel/platforms/os:darwin": ["darwin-x86/bin/m4"],
         "//build/bazel/platforms/os:linux": ["linux-x86/bin/m4"],
         "//build/bazel/platforms/os_arch:linux_musl_x86": ["linux_musl-x86/bin/m4"],
-        "//conditions:default": None,
-    })
+    }),
+    target_compatible_with = select({
+        "//build/bazel/platforms/os:darwin": [],
+        "//build/bazel/platforms/os:linux": [],
+        "//build/bazel/platforms/os_arch:linux_musl_x86": [],
+        "//conditions:default": ["@platforms//:incompatible"],
+    }),
 )
-