build: Fix darwin build
Fixes the following on darwin:
error: unknown type name 'NSString'; did you mean 'GString'?
and
Undefined symbols for architecture arm64:
"__gspell_osx_get_preferred_spell_language", referenced from:
_gspell_language_get_default in gspell-language.c.o
ld: symbol(s) not found for architecture arm64
clang-16: error: linker command failed with exit code 1 (use -v to see invocation)
by restoring https://gitlab.gnome.org/GNOME/gspell/-/blob/1.12.2/gspell/Makefile.am#L148-L161.