Work around stlport build bug.

Older versions of Android using stlport were failing to compile with an
"assert has been defined before inclusion of assert.h header" error.

Change-Id: If8e13eb08c35a34d082d99f8f57188bc55697f59
Reviewed-on: https://swiftshader-review.googlesource.com/11270
Tested-by: Nicolas Capens <nicolascapens@google.com>
Reviewed-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>