Fix a few -pedantic warnings

We're not enabling -pedantic by default, but a quick test brought up a few
issues that we should fix.
diff --git a/shared/image-loader.c b/shared/image-loader.c
index 21b20a2..a3218df 100644
--- a/shared/image-loader.c
+++ b/shared/image-loader.c
@@ -334,7 +334,7 @@
 
 
 struct image_loader {
-	char header[4];
+	unsigned char header[4];
 	int header_size;
 	pixman_image_t *(*load)(FILE *fp);
 };