Export malloc_backtrace

Bug: 27208635
(cherry picked from commit 2d4721c0c57fe2f7c1e1b40df4763a561b3cf856)

Change-Id: Ida926cabb706dfd962b692ba1775da332f68488f
diff --git a/libc/malloc_debug/exported64.map b/libc/malloc_debug/exported64.map
index 8c47449..1a6b30f 100644
--- a/libc/malloc_debug/exported64.map
+++ b/libc/malloc_debug/exported64.map
@@ -9,6 +9,7 @@
     debug_iterate;
     debug_mallinfo;
     debug_malloc;
+    debug_malloc_backtrace;
     debug_malloc_disable;
     debug_malloc_enable;
     debug_malloc_usable_size;