MAINT: fix maybe-uninitialized compiler warning in amos.h

[rgommers]

Warning this fixes with GCC 13.3.0 in a SciPy CI job:

  [649/1264] Compiling C++ object scipy/special/_ufuncs.cpython-312-x86_64-linux-gnu.so.p/xsf_wrappers.cpp.o
  In file included from ../subprojects/xsf/include/xsf/amos.h:3,
                   from ../subprojects/xsf/include/xsf/airy.h:3,
                   from ../scipy/special/xsf_wrappers.cpp:2:
  In function ‘void xsf::amos::unik(std::complex<double>, double, int, int, double, int*, std::complex<double>*, std::complex<double>*, std::complex<double>*, std::complex<double>*, std::complex<double>*)’,
      inlined from ‘int xsf::amos::unk1(std::complex<double>, double, int, int, int, std::complex<double>*, double, double, double)’ at ../subprojects/xsf/include/xsf/amos/amos.h:5716:17:
  ../subprojects/xsf/include/xsf/amos/amos.h:5379:9: warning: ‘initd’ may be used uninitialized [-Wmaybe-uninitialized]
   5379 |         if (*init == 0) {
        |         ^~
  ../subprojects/xsf/include/xsf/amos/amos.h: In function ‘int xsf::amos::unk1(std::complex<double>, double, int, int, int, std::complex<double>*, double, double, double)’:
  ../subprojects/xsf/include/xsf/amos/amos.h:5476:96: note: ‘initd’ was declared here
   5476 |         int i, ib, iflag = 0, ifn, il, inu, iuf, k, kdflg, kflag, kk, m, nw, nz, j, jc, ipard, initd, ic;
        |     

[rgommers]

The Windows failures are visible on other PRs as well, so are unrelated.

[steppi]

Thanks for fixing this @rgommers!