examples : set the C++ standard to C++17 for server (#3261)

This commit updates the server example to use C++17 as the standard.

The motivation for this change is that currently the ci-run
`ggml-100-mac-m4` is failing when compiling the server example on
macOS. The `talk-llama` example also has this setting so it looks like
an alright change to make.

ggml-ci

Refs: https://github.com/ggml-org/ci/tree/results/whisper.cpp/2a/4d6db7d90899aff3d58d70996916968e4e0d27/ggml-100-mac-m4

Author: danbev

examples/server/CMakeLists.txt

@@ -1,3 +1,6 @@
+set(CMAKE_CXX_STANDARD 17)
+set(CMAKE_CXX_STANDARD_REQUIRED ON)
+
 set(TARGET whisper-server)
 add_executable(${TARGET} server.cpp httplib.h)