pgsql: headerscheck: Use ICU_CFLAGS

headerscheck: Use ICU_CFLAGS

Otherwise, headerscheck will fail if the ICU headers are in a location
not reached by the normal CFLAGS/CPPFLAGS:

../src/include/utils/pg_locale.h:21:10: fatal error: unicode/ucol.h: No such file or directory

Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://www.postgresql.org/message-id/flat/1127775.1754417387%40sss.pgh.pa.us

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/664e0d678965719fe0d385aa3a8f2777058b119b

Modified Files
--------------
src/tools/pginclude/headerscheck | 5 +++--
1 file changed, 3 insertions(+), 2 deletions(-)