Fix build failure with newer ICU

Fix build failure with newer ICU

ICU 69 causes compile failures with PostgreSQL 10.  ICU 69 has
switched to using stdbool.h, which conflicts with the home-made
definitions that we used until PostgreSQL 10.  Newer PostgreSQL major
versions are therefore not affected.  (Older PostgreSQL versions don't
have ICU support.)

The workaround is to undefine "bool" after including ICU headers,
similar to the solution already in place for plperl for the same
underlying reasons.

Discussion: https://www.postgresql.org/message-id/flat/28588e5a-c204-0361-01f1-a1ee1b590233%40enterprisedb.com