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/include/malloc.h b/libc/include/malloc.h
index 8875e54..eaedc49 100644
--- a/libc/include/malloc.h
+++ b/libc/include/malloc.h
@@ -33,7 +33,7 @@
 extern void free(void* p);
 
 extern void* memalign(size_t alignment, size_t byte_count) __mallocfunc __wur;
-extern size_t malloc_usable_size(void* p);
+extern size_t malloc_usable_size(const void* p);
 
 extern void* valloc(size_t byte_count) __mallocfunc __wur;
 extern void* pvalloc(size_t byte_count) __mallocfunc __wur;