compiler: error on unused imports

vlib/glfw/glfw.v

@@ -4,6 +4,8 @@
 
 module glfw
 
+// note: we might need special case for this
+// see TmpGlImportHack below (joe-c)
 import gl
 
 #flag -I @VROOT/thirdparty/glfw
@@ -41,6 +43,11 @@ const (
 	KeyDown  = 264
 )
 
+// joe-c: fix & remove
+struct TmpGlImportHack {
+	hack gl.TmpGlImportHack
+}
+
 struct WinCfg {
 	width      int
 	height     int