sdl_ttf: needs freetype

Update help text while we're at it.

Based on patch by artemys. Closes #4154.

package/sdl_ttf/Config.in

@@ -1,6 +1,10 @@
 config BR2_PACKAGE_SDL_TTF
 	bool "SDL_TTF"
 	default n
+	select BR2_PACKAGE_FREETYPE
 	help
+	  SDL_ttf is a sample TrueType font library. It allows you to
+	  use TrueType fonts in your SDL applications.
 
 	  http://www.libsdl.org/projects/SDL_ttf/
+