Fix linker warning about _utf8_countTrailBytes_46

The warning:

    sqlite3.lib(sqlite.sqlite3.obj) : warning LNK4049: locally defined symbol _utf8_countTrailBytes_46 imported

This means that this symbol is decorated with __declspec(dllimport),
which is bad. Defining U_UTF8_IMPL fixes this problem, but causes
another error:

    third_party\icu\public\common\unicode\utf8.h(62) : error C2734: 'utf8_countTrailBytes_46' : const object must be initialized if not extern

For some reason ICU doesn't use U_CFUNC when exporting
utf8_countTrailBytes, but does use it when defining it or importing it.
Adding U_CFUNC fixes the above error, because it marks the symbol
`extern`.