weston-egl: Allow compilation against older EGL stack

Provide a definition of EGL_BUFFER_AGE_EXT so that you can compile against an
EGL stack that may not yet support that extension.
diff --git a/src/weston-egl-ext.h b/src/weston-egl-ext.h
index 1d4b76b..fe02368 100644
--- a/src/weston-egl-ext.h
+++ b/src/weston-egl-ext.h
@@ -58,4 +58,8 @@
 #define EGL_TEXTURE_EXTERNAL_WL         0x31DA
 #endif
 
+#ifndef EGL_BUFFER_AGE_EXT
+#define EGL_BUFFER_AGE_EXT              0x313D
+#endif
+
 #endif