llvmpipe: Don't build lp_test_round when using MSVC.

lp_test_round uses the math functions round and trunc, which aren't
available with MSVC.

Fixes the MSVC build for now.

src/gallium/drivers/llvmpipe/SConscript

@@ -80,10 +80,12 @@ if env['platform'] != 'embedded':
         'blend',
         'conv',
 	'printf',
-	'round',
 	'sincos',
     ]
 
+    if not msvc:
+        tests.append('round')
+
     for test in tests:
         target = env.Program(
             target = 'lp_test_' + test,