Fix Linux build by adding missing linker library.

The Xinerama library was added in glfw/glfw@4918514eafd620f63916c62c82a5c67636f64848 and appears to be a mandatory library for both non-Wayland and Wayland paths.

Fixes the following issue:

```bash
ubuntu@ubuntu:~$ go get -u github.com/shurcooL/glfw3
# github.com/shurcooL/glfw3
/tmp/go-build359803230/github.com/shurcooL/glfw3/_obj/glfw_x11_init.o: In function `initExtensions':
MyGoWorkspace/src/github.com/shurcooL/glfw3/glfw/src/x11_init.c:538: undefined reference to `XineramaQueryExtension'
MyGoWorkspace/src/github.com/shurcooL/glfw3/glfw/src/x11_init.c:542: undefined reference to `XineramaIsActive'
/tmp/go-build359803230/github.com/shurcooL/glfw3/_obj/glfw_x11_monitor.o: In function `_glfwPlatformGetMonitors':
MyGoWorkspace/src/github.com/shurcooL/glfw3/glfw/src/x11_monitor.c:218: undefined reference to `XineramaQueryScreens'
collect2: error: ld returned 1 exit status
```

Tested (the non-Wayland path) on a fresh VM with Ubuntu 14.04.

I didn't test the Wayland path, but it looks like the flag is needed for both non-Wayland and Wayland paths.