Export malloc_backtrace

Change-Id: Ic1adb4dfd86b9ca698443a36263a3df2c91edda3
diff --git a/libc/libc.map.txt b/libc/libc.map.txt
index 1f4da93..c000fb1 100644
--- a/libc/libc.map.txt
+++ b/libc/libc.map.txt
@@ -630,6 +630,7 @@
     madvise;
     mallinfo;
     malloc;
+    malloc_backtrace;
     malloc_disable;
     malloc_enable;
     malloc_iterate;