Update xmake.lua (#762)

packages/o/openmp/xmake.lua

@@ -73,7 +73,7 @@ package("openmp")
         return (result.cflags or result.cxxflags) and result
     end)
 
-    on_install("linux", "macosx", "windows", function (package)
+    on_install("linux", "macosx", "windows", "mingw@msys", function (package)
         -- we need not install anything because we need only compiler flags and deps
     end)