tests/tcg/i386: Build fix for hello-i386

We have -Werror=missing-prototype, add a dummy prototype to avoid that
warning.

Signed-off-by: Fam Zheng <famz@redhat.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>

tests/tcg/i386/hello-i386.c

@@ -20,6 +20,7 @@ static inline int write(int fd, const char * buf, int len)
   return status;
 }
 
+void _start(void);
 void _start(void)
 {
     write(1, "Hello World\n", 12);