xf86drm: fix sign-compare warning

xf86drm.c:3601:21: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
     while (expected < sizeof(match)) {
                     ^

Signed-off-by: Grazvydas Ignotas <notasas@gmail.com>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>
diff --git a/xf86drm.c b/xf86drm.c
index 2e8c956..b5eeeb0 100644
--- a/xf86drm.c
+++ b/xf86drm.c
@@ -3582,7 +3582,7 @@
     FILE *f;
     char buf[512];
     static const char match[9] = "\nDEVNAME=";
-    int expected = 1;
+    size_t expected = 1;
 
 
     if (fstat(fd, &sbuf))