Eliminate "-Wsign-compare" warnings on Linux ...

With these being newly enabled for the loader, layers, and demos (and it turns
out, for the icd/common directory), it is appropriate to fix them.
diff --git a/icd/common/icd-enumerate-drm.c b/icd/common/icd-enumerate-drm.c
index f65ceca..fbb00d7 100644
--- a/icd/common/icd-enumerate-drm.c
+++ b/icd/common/icd-enumerate-drm.c
@@ -186,7 +186,7 @@
 
     while (dev) {
         struct icd_drm_device *next = dev->next;
-        int i;
+        size_t i;
 
         for (i = 0; i < ARRAY_SIZE(dev->minors); i++)
             udev_device_unref((struct udev_device *) dev->minors[i]);