ext/psych/extconf.rb: braced VPATH is for nmake only

ext/psych/extconf.rb

@@ -16,9 +16,9 @@ if enable_config("bundled-libyaml", false) || !(find_header('yaml.h') && find_li
   $srcs = Dir.glob("#{$srcdir}/{,yaml/}*.c").map {|n| File.basename(n)}.sort
 
   header = 'yaml/yaml.h'
+  header = "{$(VPATH)}#{header}" if $nmake
   if have_macro("_WIN32")
     $CPPFLAGS << " -DYAML_DECLARE_STATIC -DHAVE_CONFIG_H"
-    header = "{$(VPATH)}#{header}"
   end
 
   have_header 'dlfcn.h'