This avoids issues linking to a DLL version of libspeex, since the
libspeex headers lack proper dllimport declarations for the data
symbols.

This isn't an issue when building with mingw with GNU binutils, since
GNU ld can fix up that kind of data import automatically.

libspeexdec.c already uses speex_lib_get_mode as well.
Signed-off-by: Martin Storsjö <martin@martin.st>