BUG #16791: Compile error with new ICU 68.2 for 10.15

The following bug has been logged on the website:

Bug reference: 16791
Logged by: Yury Zhuravlev
Email address: stalkerg@gmail.com
PostgreSQL version: 10.15
Operating system: Gentoo Linux
Description:

Hello,

Basically, it's a same issue as here
/messages/by-id/7a6f3336165bfe3ca66abcda7966f9d0@stz-bg.com
but latest small fix not really help for 10.15. In the same time 13.1 I can
build without issue.
Log:
```
pg_collation.c:47:1: error: conflicting types for ‘CollationCreate’
47 | CollationCreate(const char *collname, Oid collnamespace,
| ^~~~~~~~~~~~~~~
In file included from pg_collation.c:25:
../../../src/include/catalog/pg_collation_fn.h:17:12: note: previous
declaration of ‘CollationCreate’ was here
17 | extern Oid CollationCreate(const char *collname, Oid
collnamespace,
| ^~~~~~~~~~~~~~~
pg_collation.c: In function ‘CollationCreate’:
pg_collation.c:171:41: warning: passing argument 3 of ‘heap_form_tuple’ from
incompatible pointer type [-Wincompatible-pointer-types]
171 | tup = heap_form_tuple(tupDesc, values, nulls);
| ^~~~~
| |
| _Bool *
In file included from pg_collation.c:19:
../../../src/include/access/htup_details.h:802:26: note: expected ‘bool *’
{aka ‘char *’} but argument is of type ‘_Bool *’
802 | Datum *values, bool *isnull);
| ~~~~~~^~~~~~
make[3]: *** [<builtin>: pg_collation.o] Error 1
make[3]: Leaving directory
'/var/tmp/portage/dev-db/postgresql-10.15/work/postgresql-10.15/src/backend/catalog'
make[2]: *** [common.mk:41: catalog-recursive] Error 2
```

Thanks.