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>

tests/unittests/BUILD.gn

diff --git a/tests/unittests/BUILD.gn b/tests/unittests/BUILD.gn
index 10f5fa5..7cb1cc1 100644
--- a/tests/unittests/BUILD.gn
+++ b/tests/unittests/BUILD.gn
@@ -31,7 +31,11 @@
 
   include_dirs = [ "../../include" ]  # Khronos headers
 
-  defines = [ "GL_GLEXT_PROTOTYPES" ]
+  defines = [
+    "GL_GLEXT_PROTOTYPES",
+    "GL_APICALL=",
+    "GLAPI=",
+  ]
 
   # Make sure we're loading SwiftShader's libraries, not ANGLE's or the system
   # provided ones. On Windows an explicit LoadLibrary("swiftshader\lib*.dll")