linux-user: Include tcg.h in syscall.c

Code movement in an upcoming patch will show that this file
was implicitly depending on tcg.h being included indirectly.
Tested-by: NPhilippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: NPhilippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: NAlex Bennée <alex.bennee@linaro.org>
Signed-off-by: NRichard Henderson <richard.henderson@linaro.org>

linux-user/syscall.c

@@ -115,6 +115,7 @@
 #include "user/syscall-trace.h"
 #include "qapi/error.h"
 #include "fd-trans.h"
+#include "tcg.h"
 
 #ifndef CLONE_IO
 #define CLONE_IO                0x80000000      /* Clone io context */