Export malloc_backtrace

Change-Id: Ic1adb4dfd86b9ca698443a36263a3df2c91edda3
diff --git a/libc/libc.arm.brillo.map b/libc/libc.arm.brillo.map
index 1da85ea..2c0ab15 100644
--- a/libc/libc.arm.brillo.map
+++ b/libc/libc.arm.brillo.map
@@ -627,6 +627,7 @@
     madvise;
     mallinfo;
     malloc;
+    malloc_backtrace;
     malloc_disable;
     malloc_enable;
     malloc_iterate;