ACPI: Fix a warning of discarding qualifiers from pointer target type

drivers/acpi/ec.c: In function `acpi_ec_ecdt_probe':
drivers/acpi/ec.c:873: warning: passing arg 1 of `acpi_get_devices' discards qualifiers from pointer target type

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Len Brown <len.brown@intel.com>
diff --git a/include/acpi/acpixf.h b/include/acpi/acpixf.h
index b5cca5d..3d7ab9e 100644
--- a/include/acpi/acpixf.h
+++ b/include/acpi/acpixf.h
@@ -130,7 +130,7 @@
 		    void *context, void **return_value);
 
 acpi_status
-acpi_get_devices(char *HID,
+acpi_get_devices(const char *HID,
 		 acpi_walk_callback user_function,
 		 void *context, void **return_value);