jsoncpp: build with C++17 to enable string_view API

jsoncpp: build with C++17 to enable string_view API

jsoncpp 1.9.7 added std::string_view overloads for Value::get() and
Value::operator[], but these are only compiled when C++17 is active.
Building with the default C++11 standard leaves those symbols out of
the library, causing link failures for consumers that include the
headers with C++17 enabled (e.g. upmpdcli 1.9.17, domoticz 2025.2).

Add -Dcpp_std=c++17 to the meson args so the string_view API is
available in the installed library.

Signed-off-by: Alexandru Ardelean <redacted>