ACPI: suppress compiler warning in battery.c

This patch fixes following compiler warning when build via make W=1:

drivers/acpi/battery.c:149:52: warning: no previous prototype for ‘acpi_battery_present’ [-Wmissing-prototypes]
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

drivers/acpi/battery.c

@@ -146,7 +146,7 @@ struct acpi_battery {
 
 #define to_acpi_battery(x) container_of(x, struct acpi_battery, bat)
 
-inline int acpi_battery_present(struct acpi_battery *battery)
+static inline int acpi_battery_present(struct acpi_battery *battery)
 {
 	return battery->device->status.battery_present;
 }