target/xtensa: Remove duplicate typedef of DisasContext

Some older versions of gcc complain if a typedef is defined twice:

target/xtensa/translate.c:81: error: redefinition of typedef 'DisasContext'
target/xtensa/cpu.h:339: note: previous declaration of 'DisasContext' was here

Remove the now-redundant typedef from the definition of the struct in
translate.c.

Reported-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Message-id: 1515762528-22818-1-git-send-email-peter.maydell@linaro.org