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

This fixes double-definitions in linux-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: Riku Voipio <riku.voipio@linaro.org>
diff --git a/linux-user/qemu.h b/linux-user/qemu.h
index ba5b433..26b0ba2 100644
--- a/linux-user/qemu.h
+++ b/linux-user/qemu.h
@@ -13,7 +13,7 @@
 
 #include "exec/user/thunk.h"
 #include "syscall_defs.h"
-#include "syscall.h"
+#include "target_syscall.h"
 #include "exec/gdbstub.h"
 #include "qemu/queue.h"