Enable Windows build for libmodpb64

This is working towards supporting a Windows build of
libandroid_runtime.

Bug: 340885449
Test: build libmodpb64 for Windows
Change-Id: I753d927f95c4bb4347dd2d07a03461f640a037d4
diff --git a/Android.bp b/Android.bp
index 84adfba..477ae1d 100644
--- a/Android.bp
+++ b/Android.bp
@@ -49,6 +49,11 @@
         "com.android.btservices",
         "com.android.nfcservices",
     ],
+    target: {
+        windows: {
+            enabled: true,
+        },
+    },
 }
 
 cc_library_static {