Export CPR in pkg-config when HTTP support is enabled

[mormj]

This updates the generated gnuradio4.pc so builds that consume gnuradio4 through pkg-config also pick up cpr when GR_ENABLE_HTTP is enabled.

Today libgnuradio-core.a can contain HTTP/file I/O code paths that reference cpr, but the pkg-config export only advertises the core GNU Radio libraries. Downstream projects using pkg_check_modules(gnuradio4 ...) can therefore compile successfully and then fail at link time with unresolved cpr::... symbols.

This change appends -lcpr to GR4_PKGCONFIG_EXTRA_LIBS when HTTP support is active, so the generated gnuradio4.pc reflects the actual link requirements of the installed static archive.

• Fixes downstream link failures for pkg-config consumers of gnuradio4
• Makes the pkg-config export consistent with the enabled feature set
• Brings the pkg-config contract closer to the effective CMake link interface
• No change when HTTP support is disabled
• When HTTP support is enabled, pkg-config --libs gnuradio4 now includes -lcpr so downstream consumers link cleanly