Fix the unittests Ninja build.

Fix the unittests Ninja build.

On Windows, GLAPI defaults to __declspec(dllimport), while GLAPICALL is
empty, leading to declaration mismatches. Either should be empty for
the unit tests since we link statically, for all platforms.

Change-Id: I2dde0ca35108678fd9767895e0120ee4b0d7030f
Reviewed-on: https://swiftshader-review.googlesource.com/19128
Tested-by: Nicolas Capens <nicolascapens@google.com>
Reviewed-by: Alexis Hétu <sugoi@google.com>