film_grain: include config.h before other headers

Fixes warnings about redefinition of _WIN32_WINNT on Windows targets

src/film_grain_tmpl.c

@@ -26,6 +26,8 @@
  * SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
  */
 
+#include "config.h"
+
 #include <assert.h>
 #include <stdint.h>