Use treble_linker_namespace instead of treble.

More specific. Treble is being removed.

Bug: 62019611
Test: manual
Change-Id: I94c21c6a5cb6a258bfb36a0b7da23abdc73a67fb
diff --git a/libc/bionic/__bionic_get_shell_path.cpp b/libc/bionic/__bionic_get_shell_path.cpp
index 41162e9..1352815 100644
--- a/libc/bionic/__bionic_get_shell_path.cpp
+++ b/libc/bionic/__bionic_get_shell_path.cpp
@@ -40,7 +40,7 @@
    * in $PATH for the vendor shell, simply return the system shell.
    */
 
-#ifdef __ANDROID_TREBLE__
+#ifdef TREBLE_LINKER_NAMESPACES
   /* look for /system or /vendor prefix */
   char exe_path[strlen(VENDOR_PREFIX)];
   ssize_t len = readlink("/proc/self/exe", exe_path, sizeof(exe_path));