gdb/microblaze-tdep.c: fix -Wunused-but-set-variable

Fix this, seen when building with clang 14:

      CXX    microblaze-tdep.o
    /home/simark/src/binutils-gdb/gdb/microblaze-tdep.c:207:7: error: variable 'flags' set but not used [-Werror,-Wunused-but-set-variable]
      int flags = 0;
          ^

Change-Id: I59f726ed33e924912748bc475b6fd9a9394fc0d0