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]);
diff --git a/icd/common/icd-log.c b/icd/common/icd-log.c
index 3b93051..8452002 100644
--- a/icd/common/icd-log.c
+++ b/icd/common/icd-log.c
@@ -169,7 +169,7 @@
     int ret;
 
     ret = vsnprintf(msg, sizeof(msg), format, ap);
-    if (ret >= sizeof(msg) || ret < 0)
+    if ((ret >= (int) sizeof(msg)) || ret < 0)
         msg[sizeof(msg) - 1] = '\0';
 
     icd_log_str(msg_type, validation_level, src_object,