gl: add a comment for linux installation

OpenGL ES is not installed by default on linux distributions, which
can lead to a cryptic error. Add a comment after the #include
mentioning what packages contain OpenGL ES on Ubuntu, which is
helpfully printed in the error message. For example:

	$ go build golang.org/x/mobile/example/basic
	# golang.org/x/mobile/gl
	In file included from ../../mobile/gl/work.go:21:0:
	work.h:6:117: fatal error: GLES2/gl2.h: No such file or directory
	 #include <GLES2/gl2.h> // install on Ubuntu with: sudo apt-get install libegl1-mesa-dev libgles2-mesa-dev libx11-dev

Change-Id: Ia7ba583cee4dfdeed408f582b29da90de9a4fab3
Reviewed-on: https://go-review.googlesource.com/18058
Reviewed-by: Hyang-Ah Hana Kim <hyangah@gmail.com>

app/x11.go

@@ -10,9 +10,6 @@ package app
 Simple on-screen app debugging for X11. Not an officially supported
 development target for apps, as screens with mice are very different
 than screens with touch panels.
-
-On Ubuntu 14.04 'Trusty', you may have to install these libraries:
-sudo apt-get install libegl1-mesa-dev libgles2-mesa-dev libx11-dev
 */
 
 /*

gl/work.h

@@ -3,7 +3,7 @@
 // license that can be found in the LICENSE file.
 
 #ifdef os_linux
-#include <GLES2/gl2.h>
+#include <GLES2/gl2.h> // install on Ubuntu with: sudo apt-get install libegl1-mesa-dev libgles2-mesa-dev libx11-dev
 #endif
 #ifdef os_ios
 #include <OpenGLES/ES2/glext.h>