Export malloc_backtrace

Bug: 27208635
(cherry picked from commit 2d4721c0c57fe2f7c1e1b40df4763a561b3cf856)

Change-Id: Ida926cabb706dfd962b692ba1775da332f68488f
diff --git a/libc/libc.x86_64.map b/libc/libc.x86_64.map
index c7aca4f..63c82e5 100644
--- a/libc/libc.x86_64.map
+++ b/libc/libc.x86_64.map
@@ -552,6 +552,7 @@
     madvise;
     mallinfo;
     malloc;
+    malloc_backtrace;
     malloc_disable;
     malloc_enable;
     malloc_iterate;