backend/native: Fix compiler warning w/out EGL device
When building without EGL device support, the following compiler warning is seen:
src/backends/native/meta-renderer-native.c:2637:20: warning: unused
variable ‘cogl_renderer_egl’ [-Wunused-variable]
Fix the warning by placing the relevant variable declarations within a
#ifdef HAVE_EGL_DEVICE/#endif
statement.