include: fix typedefs

Use what was in the upstream header.  The switch of XGL_SIZE to size_t
generates quite some warnings and is fixed.
diff --git a/icd/common/icd-format.h b/icd/common/icd-format.h
index 1bfc8af..963eb61 100644
--- a/icd/common/icd-format.h
+++ b/icd/common/icd-format.h
@@ -97,7 +97,7 @@
     return (icd_format_is_compressed(format)) ? 4 : 1;
 }
 
-XGL_SIZE icd_format_get_size(XGL_FORMAT format);
+size_t icd_format_get_size(XGL_FORMAT format);
 
 XGL_UINT icd_format_get_channel_count(XGL_FORMAT format);