Meant to use an "#if" not "#if defined" for USE_HEADLESS_SURFACE. Before
this fix, we would always request the headless surface extension, which
would usually fail on Windows if no loader is installed since current
GPU drivers don't implement this extension yet.

Change-Id: I5bcf9e08f467fd85712d1bc3504e3f251e886664
Bug: b/176981107
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/53208
Kokoro-Result: kokoro <noreply+kokoro@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>
Tested-by: Antonio Maiorano <amaiorano@google.com>
Commit-Queue: Antonio Maiorano <amaiorano@google.com>