minui: do not use flexible array initialization

Avoiding the use of gnu extensions improves code portability

Change-Id: I441a49ce08b6ecb1d735ca453552fa92026c5016
diff --git a/minui/font_10x18.h b/minui/font_10x18.h
index 7f96465..29d7053 100644
--- a/minui/font_10x18.h
+++ b/minui/font_10x18.h
@@ -3,7 +3,7 @@
   unsigned height;
   unsigned cwidth;
   unsigned cheight;
-  unsigned char rundata[];
+  unsigned char rundata[2973];
 } font = {
   .width = 960,
   .height = 18,