Interface with health HAL to read battery voltage

Bug: 78793464
Test: fastboot getvar battery-voltage
Change-Id: Ie0763e4f08327ec7649c5629066eb73e8142e0e6
diff --git a/fastboot/device/variables.h b/fastboot/device/variables.h
index b06881e..e7c3c7c 100644
--- a/fastboot/device/variables.h
+++ b/fastboot/device/variables.h
@@ -55,7 +55,8 @@
 bool GetVariant(FastbootDevice* device, const std::vector<std::string>& args, std::string* message);
 bool GetOffModeChargeState(FastbootDevice* device, const std::vector<std::string>& args,
                            std::string* message);
-
+bool GetBatteryVoltage(FastbootDevice* device, const std::vector<std::string>& args,
+                       std::string* message);
 // Helpers for getvar all.
 std::vector<std::vector<std::string>> GetAllPartitionArgsWithSlot(FastbootDevice* device);
 std::vector<std::vector<std::string>> GetAllPartitionArgsNoSlot(FastbootDevice* device);