cmake: fixes for windows

cmake/example.cpp

Index: cmake/example.cpp
diff --git a/cmake/example.cpp b/cmake/example.cpp
index 20259b558928bdf05dde2b53d53c75402193c976..38ce901450916b2640f0f89b52b3951da59b959d 100644
--- a/cmake/example.cpp
+++ b/cmake/example.cpp
@@ -60,7 +60,7 @@ static std::shared_ptr<SkSurface> create_opengl_surface(int w, int h) {
 
 int main(int, char**) {
     bool gl_ok = setup_gl_context();
-    srand(time(nullptr));
+    srand((unsigned)time(nullptr));
     std::shared_ptr<SkSurface> surface = (gl_ok && rand() % 2) ? create_opengl_surface(320, 240)
                                                                : create_raster_surface(320, 240);