Fix no-MPB stub for get_eigenmode to match updated declaration

[Luochenghuang]

PR #3167 added cache_dispersive and cache_frequency parameters to fields::get_eigenmode in meep.hpp, but only updated the no-MPB stub for get_eigenmode_coefficients. The get_eigenmode stub was missed. This causes a compile error when building without MPB (e.g., the ASAN and UBSAN CI jobs):

mpb.cpp:1075:15: error: out-of-line definition of 'get_eigenmode' does not match any declaration in 'meep::fields'
 1075 | void *fields::get_eigenmode(double frequency, direction d, const volume where, const volume eig_vol,
      |               ^~~~~~~~~~~~~

This PR adds the two missing parameters to the no-MPB stub signature.
This PR was tested against the sanitizers CI here.

[Luochenghuang]

Tagging @stevengj.