linux-user/flatload.c: Use "" for include of QEMU header target_flat.h

linux-user/flatload.c: Use "" for include of QEMU header target_flat.h

The target_flat.h file is a QEMU header, so we should include it using
quotes, not angle brackets.

Coverity otherwise is unable to find the header:

"../linux-user/flatload.c", line 40: error #1712: cannot open source file
          "target_flat.h"
  #include <target_flat.h>
                          ^

because the relevant directory is only on the -iquote path, not the -I path.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-id: 20200319193323.2038-5-peter.maydell@linaro.org