fix(runtime): source c ftplugin properly for cpp on Windows (#29053)

On Windows, '{' is currently not treated as a wildcard char, so another
wildcard char is needed for the pattern to be treated as a wildcard.

It may be worth trying to make '{' always a wildcard char in the future,
but that'll be a bit harder as it'll be necessary to make sure '{' is
escaped at various places.

test/functional/lua/runtime_spec.lua

@@ -401,4 +401,12 @@ describe('runtime:', function()
       eq('ABab', eval('g:seq'))
     end)
   end)
+
+  it('cpp ftplugin loads c ftplugin #29053', function()
+    eq('', eval('&commentstring'))
+    eq('', eval('&omnifunc'))
+    exec('edit file.cpp')
+    eq('/*%s*/', eval('&commentstring'))
+    eq('ccomplete#Complete', eval('&omnifunc'))
+  end)
 end)