sched.h needs to be imported on Darwin/OSX targets.

sched_yield is used but the include reference on Darwin is missing. This patch
conditionally guards on Darwin/OSX to import sched.h first.

Reviewed-by: Jeremy Huddleston Sequoia <jeremyhu@apple.com>
Reviewed-by: Bruce Cherniak <bruce.cherniak@intel.com>

src/gallium/drivers/swr/swr_fence.cpp

@@ -29,6 +29,10 @@
 #include "swr_screen.h"
 #include "swr_fence.h"
 
+#ifdef __APPLE__
+#include <sched.h>
+#endif
+
 #if defined(PIPE_CC_MSVC) // portable thread yield
    #define sched_yield SwitchToThread
 #endif