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
1 file changed