build: don't use explicitly relative include paths

GCC has problems canonicalising some paths.

object.cpp

@@ -7,11 +7,11 @@
  *      2016-2017, Danny Robson <danny@nerdcruft.net>
  */
 
-#include "./object.hpp"
+#include "object.hpp"
 
-#include "./instance.hpp"
+#include "instance.hpp"
 
-#include "./physical_device.hpp"
+#include "physical_device.hpp"
 
 using cruft::vk::object;
 using cruft::vk::enumerated;

object.hpp

@@ -10,8 +10,8 @@
 #ifndef CRUFT_VK_OBJECT_HPP
 #define CRUFT_VK_OBJECT_HPP
 
-#include "./traits.hpp"
-#include "./except.hpp"
+#include "traits.hpp"
+#include "except.hpp"
 
 #include <utility>
 #include <vector>