icu: export C API on MinGW

[mmuetzel]

Probably fixes #24493.

[mmuetzel]

Apparently, this change leads to a lot of symbols that are no longer being exported from the shared libraries. But the missing symbols from #24493 are exported from libicuin77.dll now.

I checked a few of the symbol names that are no longer exported. And afaict, they corresponded to internal functions (that aren't defined in the installed headers and don't have API attributes in the sources).
Also, the package grokker didn't detect any packages that are using those symbols.

[lazka]

lgtm

In theory icu-uc.pc could be extended with Cflags.private: -DU_STATIC_IMPLEMENTATION so the dllimport is removed for "pkgconf --static" again. Not sure how likely that is that users depended on that.

[mmuetzel]

Good point!

I amended the patch to also add that line to the pkg-config files.