PCI: Constify label attribute_group structures

attribute_groups are not supposed to change at runtime.  All functions
working with attribute_groups provided by <linux/sysfs.h> work with const
attribute_group.  So mark the non-const structs as const.

File size before:
   text	   data	    bss	    dec	    hex	filename
    930	    320	      0	   1250	    4e2	drivers/pci/pci-label.o

File size After adding 'const':
   text	   data	    bss	    dec	    hex	filename
   1058	    192	      0	   1250	    4ca	drivers/pci/pci-label.o

Signed-off-by: Arvind Yadav <arvind.yadav.cs@gmail.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>

drivers/pci/pci-label.c

@@ -123,7 +123,7 @@ static struct attribute *smbios_attributes[] = {
 	NULL,
 };
 
-static struct attribute_group smbios_attr_group = {
+static const struct attribute_group smbios_attr_group = {
 	.attrs = smbios_attributes,
 	.is_visible = smbios_instance_string_exist,
 };
@@ -260,7 +260,7 @@ static struct attribute *acpi_attributes[] = {
 	NULL,
 };
 
-static struct attribute_group acpi_attr_group = {
+static const struct attribute_group acpi_attr_group = {
 	.attrs = acpi_attributes,
 	.is_visible = acpi_index_string_exist,
 };