Fix declaration of malloc_usable_size()

The function should take a 'const void*' parameter, instead of 'void*'.
Note that the implementation in upstream-dlmalloc/malloc.c already does
this.

For context, see http://b.android.com/55725

Change-Id: Iefd55cdb8996699189e0545f9195972490306227
diff --git a/libc/bionic/malloc_debug_common.cpp b/libc/bionic/malloc_debug_common.cpp
index f6fa83c..9cc84c3 100644
--- a/libc/bionic/malloc_debug_common.cpp
+++ b/libc/bionic/malloc_debug_common.cpp
@@ -190,7 +190,7 @@
     return dlmallinfo();
 }
 
-extern "C" size_t malloc_usable_size(void* mem) {
+extern "C" size_t malloc_usable_size(const void* mem) {
     return dlmalloc_usable_size(mem);
 }