platform-xml: Add platform XML dependencies on common XML

This change specifies a dependency from the platform XML packages to
the common XML package.

In the previous change, we introduced the necessary dependency from the
common-p8-xml package to the openpower-mrw and hostboot. These are
already expressed in the DEPENDENCIES variable, but need a corresponding
kconfig dep too.

Signed-off-by: Jeremy Kerr <jk@ozlabs.org>
diff --git a/openpower/package/garrison-xml/Config.in b/openpower/package/garrison-xml/Config.in
index a1626d0..a71a2c8 100755
--- a/openpower/package/garrison-xml/Config.in
+++ b/openpower/package/garrison-xml/Config.in
@@ -1,6 +1,7 @@
 config BR2_PACKAGE_GARRISON_XML
         bool "garrison_xml"
         default y if (BR2_OPENPOWER_CONFIG_NAME = "garrison")
+        select BR2_PACKAGE_COMMON_P8_XML
         help
             Utilites for building xml and the targeting binary image