swr/rast: Added missing define for Linux/gcc

+ ZeroMemory() macro definition for non win32-compilation in common/os.h

Reviewed-by: Bruce Cherniak <bruce.cherniak@intel.com>

src/gallium/drivers/swr/rasterizer/common/os.h

@@ -244,6 +244,7 @@ void AlignedFree(void* p)
 #define PRAGMA_WARNING_PUSH_DISABLE(...)
 #define PRAGMA_WARNING_POP()
 
+#define ZeroMemory(dst, size) memset(dst, 0, size)
 #else
 
 #error Unsupported OS/system.