build: use path.join() for objdir

use path.join() to set objdir.

Signed-off-by: Tomi Valkeinen <tomi.valkeinen@iki.fi>

build/premake4.lua

@@ -16,7 +16,7 @@ solution "CppSharp"
   flags { common_flags }
   
   location (builddir)
-  objdir (builddir .. "/obj/")
+  objdir (path.join(builddir, "obj"))
   targetdir (libdir)
   debugdir (bindir)