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 <51edc787c30f24c4904e8ffbb5523c3a9f7a451d@google.com>
Reviewed-by: Alexis Hétu <1c7141772178a7e80858c3db5052b328076f08f4@google.com>

bkaradzic/SwiftShader,bkaradzic/SwiftShader,bkaradzic/SwiftShader,google/swiftshader,bkaradzic/SwiftShader,google/swiftshader,google/swiftshader,bkaradzic/SwiftShader