mesa: restore _mesa_snprintf() - it's needed for Windows

This reverts part of commit 298be2b028263b2c343a707662c6fbfa18293cb2
diff --git a/src/mesa/shader/shader_api.c b/src/mesa/shader/shader_api.c
index 8bd780b..129a973 100644
--- a/src/mesa/shader/shader_api.c
+++ b/src/mesa/shader/shader_api.c
@@ -2076,7 +2076,7 @@
       unit = prog->SamplerUnits[sampler];
       target = prog->SamplerTargets[sampler];
       if (targetUsed[unit] != -1 && targetUsed[unit] != target) {
-         snprintf(errMsg, 100,
+         _mesa_snprintf(errMsg, 100,
 		  "Texture unit %d is accessed both as %s and %s",
 		  unit, targetName[targetUsed[unit]], targetName[target]);
          return GL_FALSE;