build: [bsd-user] Rename "syscall.h" to "target_syscall.h" in target directories

This fixes double-definitions in bsd-user builds when using the UST
tracing backend (which indirectly includes the system's "syscall.h").

Signed-off-by: Lluís Vilanova <vilanova@ac.upc.edu>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>

bsd-user/qemu.h

@@ -35,7 +35,7 @@ enum BSDType {
 extern enum BSDType bsd_type;
 
 #include "syscall_defs.h"
-#include "syscall.h"
+#include "target_syscall.h"
 #include "target_signal.h"
 #include "exec/gdbstub.h"