healthd: Turn on -Werror

Change-Id: I72caf38008ee7d842dfacd3f58add9bc8e4787e8
diff --git a/healthd/BatteryMonitor.cpp b/healthd/BatteryMonitor.cpp
index 9368225..7310994 100644
--- a/healthd/BatteryMonitor.cpp
+++ b/healthd/BatteryMonitor.cpp
@@ -37,7 +37,7 @@
 namespace android {
 
 struct sysfsStringEnumMap {
-    char* s;
+    const char* s;
     int val;
 };