Add gbm (generic/graphics buffer manager)

This pushes to Khronos the change done by Benjamin Franzke in Mesa
commit eddcecbf74da26716509c047b95e85b00c12bab4 "Add gbm
(generic/graphics buffer manager)" [1].

[1] https://cgit.freedesktop.org/mesa/mesa/commit/?id=eddcecbf74da26716509c047b95e85b00c12bab4
diff --git a/api/EGL/eglplatform.h b/api/EGL/eglplatform.h
index 5e1fec4..cee1471 100644
--- a/api/EGL/eglplatform.h
+++ b/api/EGL/eglplatform.h
@@ -89,6 +89,12 @@
 typedef struct wl_egl_pixmap  *EGLNativePixmapType;
 typedef struct wl_egl_window  *EGLNativeWindowType;
 
+#elif defined(__GBM__)
+
+typedef struct gbm_device  *EGLNativeDisplayType;
+typedef struct gbm_bo      *EGLNativePixmapType;
+typedef void               *EGLNativeWindowType;
+
 #elif defined(__ANDROID__) || defined(ANDROID)
 
 struct ANativeWindow;