Correct libstdc++ version

The version script is introduced in O

Bug: http://b/28987948
Change-Id: Icd0a27d1b67e8227e44aa6e771816f0e6ed489b8
diff --git a/libc/libstdc++.map.txt b/libc/libstdc++.map.txt
index d11d4c0..12e26cd 100644
--- a/libc/libstdc++.map.txt
+++ b/libc/libstdc++.map.txt
@@ -1,4 +1,4 @@
-LIBC {
+LIBC_O {
   global:
     _ZdaPv;
     _ZdaPvRKSt9nothrow_t;