games/openttd: unbreak with ICU 61

src/language.h:108:8: error: unknown type name 'Collator'; did you mean 'UCollator'?
extern Collator *_current_collator;

^~~~~~~~
UCollator

/usr/local/include/unicode/ucol.h:58:26: note: 'UCollator' declared here
typedef struct UCollator UCollator;

^

https://ssl.icu-project.org/trac/changeset/40705

PR: 227042
Reported by: antoine (via exp-run)