Everywhere else uses #ifdef, this used just #if. When the next commit
adds -Wundef to the compiler options, this #if here will start failing
as ENABLE_EGL is undefined.

It would be much better to use Meson's set10() for ENABLE_EGL and change
all #ifdef into #if, but I opted for the smaller change for now.

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.com>