fallback_malloc: silence qual-cast warning (NFC)

This silences a GCC 4.9.2 qual-cast warning in the fallback_malloc codepath.
NFC.

git-svn-id: https://llvm.org/svn/llvm-project/libcxxabi/trunk@238944 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/src/fallback_malloc.ipp b/src/fallback_malloc.ipp
index 135d489..56da36c 100644
--- a/src/fallback_malloc.ipp
+++ b/src/fallback_malloc.ipp
@@ -68,7 +68,7 @@
     { return (heap_node *) ( heap + ( offset * sizeof (heap_node))); }
 
 heap_offset offset_from_node ( const heap_node *ptr )
-    { return static_cast<heap_offset>(static_cast<size_t>(((char *) ptr ) - heap)  / sizeof (heap_node)); }
+    { return static_cast<heap_offset>(static_cast<size_t>(reinterpret_cast<const char *>(ptr) - heap)  / sizeof (heap_node)); }
  
 void init_heap () {
     freelist = (heap_node *) heap;