Fix attempting to load libX11 when rendering headless.

Bug 26776258

Change-Id: I326e76d2650b3992835d74baea159b13f14b2e3e
Reviewed-on: https://swiftshader-review.googlesource.com/4720
Tested-by: Nicolas Capens <capn@google.com>
Reviewed-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <capn@google.com>