package/flare-engine: require sdl2_image with png support

flare-engine fail to start if sdl2_image library is build without
libpng support.

Signed-off-by: Romain Naour <romain.naour@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
(cherry picked from commit 6c4328a5abae7a89103e1cd399c73dba4bbac63c)
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

package/flare-engine/Config.in

@@ -2,6 +2,7 @@ config BR2_PACKAGE_FLARE_ENGINE
 	bool "flare-engine"
 	depends on BR2_INSTALL_LIBSTDCPP
 	depends on !BR2_STATIC_LIBS # SDL2
+	select BR2_PACKAGE_LIBPNG # SDL2_IMAGE needs libpng support
 	select BR2_PACKAGE_SDL2
 	select BR2_PACKAGE_SDL2_IMAGE
 	select BR2_PACKAGE_SDL2_MIXER