hw/xfree86/os-support: explicitly define SYSCALL macro with empty while body

The `SYSCALL` macro can seldom generate a warning in Clang when used
immediately before another macro (e.g. `DebugF`). We can explicitly
define the empty body to suppress these warnings.

Signed-off-by: Spxctre <[email protected]>

Author: onlyspxctre

hw/xfree86/os-support/xf86_OSlib.h

@@ -234,7 +234,7 @@ struct pcvtid {
 #define MAP_FAILED ((void *)-1)
 #endif
 
-#define SYSCALL(call) while(((call) == -1) && (errno == EINTR))
+#define SYSCALL(call) while(((call) == -1) && (errno == EINTR)) {}
 
 #include "xf86_OSproc.h"