Add credits to gpusprites example

Author: fincs

graphics/gpu/gpusprites/source/main.c

@@ -1,3 +1,6 @@
+// Simple citro2d sprite drawing example
+// Images borrowed from:
+//   https://kenney.nl/assets/space-shooter-redux
 #include <citro2d.h>
 
 #include <assert.h>