Hide ICU C++ APIs from pg_locale.hHEADmaster

The cpluspluscheck script wraps our headers in `extern "C"`. This
disables name mangling, which is necessary for the C++ templates
in system ICU headers. cpluspluscheck thus fails when the build is
configured with ICU (the default). CI worked around this by disabling
ICU, but let's make it work so others can run the script.

We can specify we only want the C APIs by defining U_SHOW_CPLUSPLUS_API
to be 0 in pg_locale.h. Extensions that want the C++ APIs can include
ICU headers separately before including PostgreSQL headers.

ICU documentation:
https://github.com/unicode-org/icu/blob/main/docs/processes/release/tasks/healthy-code.md#test-icu4c-headers

Suggested-by: Andres Freund <[email protected]>
Reviewed-by: Peter Eisentraut <[email protected]>
Reviewed-by: Tom Lane <[email protected]>
Reviewed-by: Andres Freund <[email protected]>
Discussion: https://postgr.es/m/20220323002024.f2g6tivduzrktgfa%40alap3.anarazel.de
Discussion: https://postgr.es/m/CANWCAZbgiaz1_0-F4SD%2B%3D-e9onwAnQdBGJbhg94EqUu4Gb7WyA%40mail.gmail.com

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