Use treble_linker_namespace instead of treble.

More specific. Treble is being removed.

Bug: 62019611
Test: manual
Change-Id: I94c21c6a5cb6a258bfb36a0b7da23abdc73a67fb
diff --git a/libc/Android.bp b/libc/Android.bp
index 3ec75ee..38e3e99 100644
--- a/libc/Android.bp
+++ b/libc/Android.bp
@@ -1419,8 +1419,8 @@
         },
     },
     product_variables: {
-        treble: {
-            cflags: ["-D__ANDROID_TREBLE__"],
+        treble_linker_namespaces: {
+            cflags: ["-DTREBLE_LINKER_NAMESPACES"],
         },
     },
     cppflags: ["-Wold-style-cast"],