bp2build: mark filegroups as bp2build_available.

Test: build/bazel/scripts/milestone-2/demo.sh
Change-Id: Ife199b1860521c55baa96cadc42bd021556c62cd
diff --git a/libc/Android.bp b/libc/Android.bp
index 736c147..8d0a24b 100644
--- a/libc/Android.bp
+++ b/libc/Android.bp
@@ -2345,6 +2345,7 @@
         "kernel/uapi/linux/input.h",
         "kernel/uapi/linux/input-event-codes.h",
     ],
+    bazel_module: { bp2build_available: true },
 }
 
 // Generate a syscall name / number mapping. These objects are text files
@@ -2406,6 +2407,7 @@
 filegroup {
     name: "all_kernel_uapi_headers",
     srcs: ["kernel/uapi/**/*.h"],
+    bazel_module: { bp2build_available: true },
 }