[gdb] Add missing i18n support to warning strings (part 1)

Add missing i18n support to some warning strings.

Result of:
...
$ find gdb* -type f -name "*.[ch]" -o -name "*.cc" \
    | egrep -v /testsuite/ \
    | xargs sed -i \
        's/\([ \t]\)warning (\("[^"]*"\));/\1warning (_(\2));/'
...

Approved-By: Tom Tromey <tom@tromey.com>