contrib: libplacebo: disable package not usable on Winstore builds

uses OpenGL. Once it uses Angle and/or Vulkan it may be usable.

contrib/src/libplacebo/rules.mak

@@ -6,7 +6,9 @@ PLACEBO_URL := https://code.videolan.org/videolan/libplacebo/-/archive/v$(PLACEB
 
 DEPS_libplacebo = glslang
 
+ifndef HAVE_WINSTORE
 PKGS += libplacebo
+endif
 ifeq ($(call need_pkg,"libplacebo"),)
 PKGS_FOUND += libplacebo
 endif