gma: Justify some use-visibility warnings

These `use type` statements were required for older compilers
(<= GCC 6.2) but are warned about by newer versions, sigh.

Change-Id: I50d6ea19b5a9e0c7d839f6a9781b6179156ce162
Signed-off-by: Nico Huber <nico.h@gmx.de>
Reviewed-on: https://review.coreboot.org/18690
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>
Reviewed-by: Patrick Georgi <pgeorgi@google.com>
diff --git a/common/hw-gfx-gma-config_helpers.ads b/common/hw-gfx-gma-config_helpers.ads
index 1029177..a9cde2b 100644
--- a/common/hw-gfx-gma-config_helpers.ads
+++ b/common/hw-gfx-gma-config_helpers.ads
@@ -35,7 +35,10 @@
 
    ----------------------------------------------------------------------------
 
+   pragma Warnings (GNAT, Off, """Integer_32"" is already use-visible *",
+                    Reason => "Needed for older compiler versions");
    use type HW.Pos32;
+   pragma Warnings (GNAT, On, """Integer_32"" is already use-visible *");
    function Validate_Config
      (Framebuffer : Framebuffer_Type;
       Port_Cfg    : Port_Config;