Silence healthd warnings.

Change-Id: I339cf62ce93cb268e00ba26f1c310d0586899476
diff --git a/healthd/BatteryMonitor.cpp b/healthd/BatteryMonitor.cpp
index 688c7ff..fa87274 100644
--- a/healthd/BatteryMonitor.cpp
+++ b/healthd/BatteryMonitor.cpp
@@ -37,7 +37,7 @@
 namespace android {
 
 struct sysfsStringEnumMap {
-    char* s;
+    const char* s;
     int val;
 };