Fix build of OpenVG backend on platforms without X11

https://bugs.freedesktop.org/show_bug.cgi?id=72911

Signed-off-by: Paul Cercueil <paul@crapouillou.net>

Index: b/boilerplate/cairo-boilerplate-vg.c
===================================================================
--- a/boilerplate/cairo-boilerplate-vg.c
+++ b/boilerplate/cairo-boilerplate-vg.c
@@ -215,7 +215,7 @@ _cairo_boilerplate_vg_create_surface_egl (const char		    *name,
 	EGL_ALPHA_SIZE, 8,
 	EGL_SURFACE_TYPE, EGL_PBUFFER_BIT,
 	EGL_RENDERABLE_TYPE, EGL_OPENVG_BIT,
-	None
+	EGL_NONE
     };
     int rgb_attribs[] = {
 	EGL_RED_SIZE, 8,
@@ -225,7 +225,7 @@ _cairo_boilerplate_vg_create_surface_egl (const char		    *name,
 	EGL_VG_ALPHA_FORMAT, EGL_VG_ALPHA_FORMAT_PRE_BIT,
 	EGL_SURFACE_TYPE, EGL_PBUFFER_BIT,
 	EGL_RENDERABLE_TYPE, EGL_OPENVG_BIT,
-	None
+	EGL_NONE
     };
     int dummy_attribs[] = {
 	EGL_WIDTH, 8, EGL_HEIGHT, 8,