export EGL_PLATFORM as x11 in eglonxbackend

required because the early initEGL to detect buffer_age support
can cause usage of the wrong driver in eglGetDisplay()

BUG: 358750
FIXED-IN: 5.6
REVIEW: 126958

eglonxbackend.cpp

@@ -89,6 +89,7 @@ EglOnXBackend::~EglOnXBackend()
 
 void EglOnXBackend::init()
 {
+    qputenv("EGL_PLATFORM", "x11");
     initEGL();       // required to toggle
     initBufferAge(); // EGL_SWAP_BEHAVIOR_PRESERVED_BIT
     if (!initRenderingContext()) {