Export malloc_backtrace

Change-Id: Ic1adb4dfd86b9ca698443a36263a3df2c91edda3
diff --git a/libc/libc.x86.brillo.map b/libc/libc.x86.brillo.map
index c57a072..7ac0976 100644
--- a/libc/libc.x86.brillo.map
+++ b/libc/libc.x86.brillo.map
@@ -624,6 +624,7 @@
     madvise;
     mallinfo;
     malloc;
+    malloc_backtrace;
     malloc_disable;
     malloc_enable;
     malloc_iterate;