vulkan: Use the generated version of vkconfig.h

Build fails in ext/vulkan/xcb and ext/vulkan/wayland when:
* building from tarball
* building out-of-tree
* Only one WSI integration (xcb or wayland) is enabled by configure.ac
This is because vkconfig.h from source directory gets used instead
of the generated one.

Add the correct build directory to "-I". Use angle bracket
include in vkapi.h so that it actually looks in the include search
path instead of defaulting to the same (source tree) directory.

https://bugzilla.gnome.org/show_bug.cgi?id=784539

ext/vulkan/vkapi.h

@@ -23,7 +23,7 @@
 
 #define VK_PROTOTYPES
 
-#include "vkconfig.h"
+#include <vkconfig.h>
 #include "vk_fwd.h"
 #include "vkmacros.h"
 

ext/vulkan/wayland/Makefile.am

@@ -14,6 +14,7 @@ noinst_HEADERS = \
 
 libgstvulkan_wayland_la_CFLAGS = \
 	-I$(top_srcdir)/gst-libs \
+	-I$(top_builddir)/ext/vulkan \
 	-I$(top_srcdir)/ext/vulkan \
 	-I$(top_builddir)/gst-libs \
 	$(GST_PLUGINS_BASE_CFLAGS) \

ext/vulkan/xcb/Makefile.am

@@ -14,6 +14,7 @@ noinst_HEADERS = \
 
 libgstvulkan_xcb_la_CFLAGS = \
 	-I$(top_srcdir)/gst-libs \
+	-I$(top_builddir)/ext/vulkan \
 	-I$(top_srcdir)/ext/vulkan \
 	-I$(top_builddir)/gst-libs \
 	$(GST_PLUGINS_BASE_CFLAGS) \