odrefresh: Avoid string copy

GetFreeSpace() takes a const std::string&, so we don't need to
use c_str().

Test: TreeHugger
Change-Id: I8d7fd3841481d7aee2caa59ce775c8fe380aeb3d
diff --git a/odrefresh/odrefresh.cc b/odrefresh/odrefresh.cc
index 218e07e..f78802d 100644
--- a/odrefresh/odrefresh.cc
+++ b/odrefresh/odrefresh.cc
@@ -1229,7 +1229,7 @@
 
     uint64_t bytes_available;
     const std::string& art_apex_data_path = GetArtApexData();
-    if (!GetFreeSpace(art_apex_data_path.c_str(), &bytes_available)) {
+    if (!GetFreeSpace(art_apex_data_path, &bytes_available)) {
       return false;
     }