driver->threaded_video never used - buildfix for Windows

driver.h

@@ -226,7 +226,6 @@ typedef struct driver
    menu_handle_t *menu;
    const menu_ctx_driver_t *menu_ctx;
 #endif
-   bool threaded_video;
 
    /* If set during context deinit, the driver should keep
     * graphics context alive to avoid having to reset all 

gfx/d3d/render_chain_cg.cpp

@@ -16,6 +16,8 @@
  */
 
 #include <string.h>
+#include <math.h>
+
 #include <string>
 #include <vector>