doc: libvlc: fix QtGL shader compilation issue

This is the error I get on my Win10 machine

QOpenGLShader::compile(Fragment): ERROR: 0:12: ';' : syntax error
*** Problematic Fragment shader source code ***
#ifdef GL_KHR_blend_equation_advanced
#extension GL_ARB_fragment_coord_conventions : enable
#extension GL_KHR_blend_equation_advanced : enable
#endif
#ifndef GL_FRAGMENT_PRECISION_HIGH
#define highp mediump
#endif
#line 1
#ifdef GL_ES
precision highp float;
#endif

uniform sampler2D texture;

varying vec2 texcoord;

void main()
{
    gl_FragColor = texture2D(texture, texcoord);
};

***
QOpenGLShaderProgram::uniformLocation(texture): shader program is not linked