Fix mac build

Fix incorrect UNUSED.

Test: untested
Change-Id: Icf402d3dcf4c3c39301f1637ef316e5b1f463030
diff --git a/libartbase/base/mem_map.cc b/libartbase/base/mem_map.cc
index c417d01..d9760bd 100644
--- a/libartbase/base/mem_map.cc
+++ b/libartbase/base/mem_map.cc
@@ -365,7 +365,7 @@
 
 bool MemMap::ReplaceWith(MemMap* source, /*out*/std::string* error) {
 #if !HAVE_MREMAP_SYSCALL
-  UNUSED(source_ptr);
+  UNUSED(source);
   *error = "Cannot perform atomic replace because we are missing the required mremap syscall";
   return false;
 #else  // !HAVE_MREMAP_SYSCALL