app: fix X11 build

Change-Id: Ida7cd57eefac66bcff89720ac5c82015d1d37f00
Reviewed-on: https://go-review.googlesource.com/13540
Reviewed-by: Jeffrey Regan <jregan@google.com>
Reviewed-by: David Crawshaw <crawshaw@golang.org>

app/x11.go

@@ -78,7 +78,7 @@ func main(f func(App)) {
 func onResize(w, h int) {
 	// TODO(nigeltao): don't assume 72 DPI. DisplayWidth and DisplayWidthMM
 	// is probably the best place to start looking.
-	pixelsPerPt := 1
+	pixelsPerPt := float32(1)
 	eventsIn <- config.Event{
 		WidthPx:     w,
 		HeightPx:    h,