tee-supplicant: rpmb.c: add __attribute__((fallthrough))

tee-supplicant: rpmb.c: add __attribute__((fallthrough))

Fix the following error reported by GCC 8.1:

src/rpmb.c: In function ‘rpmb_data_req’:
src/rpmb.c:683:6: error: this statement may fall through [-Werror=implicit-fallthrough=]
   if (rsp_nfrm != 1) {
      ^
src/rpmb.c:689:2: note: here
  case RPMB_MSG_TYPE_REQ_AUTH_DATA_READ:
  ^~~~

I could not silence this warning using any of the special comments
documented on the GCC Warning Options page [1]. It is weird, because we
use -Wextra which is supposed to enable -Wimplicit-fallthrough=3, which is
supposed to recognize the comment we had already (/* Fall through */).

Link: [1] https://gcc.gnu.org/onlinedocs/gcc/Warning-Options.html
Signed-off-by: Jerome Forissier <jerome.forissier@linaro.org>
Reviewed-by: Joakim Bech <joakim.bech@linaro.org>