otel build update for GCC 15 (#12180)

When building the opentelemetry plugin against GCC 15, we get a warning
from a deprecated use of ciso646 in the library. No version of
opentelemetry-cpp currently addresses this. The `-Wno-cpp` silences
that warning. Here is the warning, for reference:

```
In file included from /opt/include/opentelemetry/nostd/internal/absl/base/options.h:73,
                 from /opt/include/opentelemetry/nostd/internal/absl/base/config.h:66,
                 from /opt/include/opentelemetry/nostd/internal/absl/types/variant.h:45,
                 from /opt/include/opentelemetry/nostd/variant.h:52,
                 from /opt/include/opentelemetry/common/attribute_value.h:10,
                 from /opt/include/opentelemetry/sdk/trace/recordable.h:6,
                 from /opt/include/opentelemetry/sdk/trace/exporter.h:9,
                 from /opt/include/opentelemetry/exporters/otlp/otlp_http_exporter.h:7,
                 from /home/bneradt/src/ts_edge/plugins/experimental/otel_tracer/tracer_common.h:18,
                 from /home/bneradt/src/ts_edge/plugins/experimental/otel_tracer/otel_tracer.cc:40:
/usr/include/c++/15/ciso646:46:4: error: #warning "<ciso646> is deprecated in C++17, use <version> to detect implementation-specific macros" [-Werror=cpp]
   46 | #  warning "<ciso646> is deprecated in C++17, use <version> to detect implementation-specific macros"
      |    ^~~~~~~
```

Author: bneradt

cmake/Findopentelemetry.cmake

@@ -103,6 +103,7 @@ if(opentelemetry_FOUND)
   if(APPLE)
     if(NOT TARGET opentelemetry::opentelemetry)
       add_library(opentelemetry::opentelemetry INTERFACE IMPORTED)
+      target_compile_options(opentelemetry::opentelemetry INTERFACE -Wno-cpp)
       target_include_directories(opentelemetry::opentelemetry INTERFACE ${opentelemetry_INCLUDE_DIRS})
       target_link_libraries(opentelemetry::opentelemetry INTERFACE ${opentelemetry_found_LIBRARIES})
       list(APPEND opentelemetry_LIBRARIES opentelemetry::opentelemetry)
@@ -118,5 +119,6 @@ if(opentelemetry_FOUND)
         list(APPEND opentelemetry_LIBRARIES opentelemetry::${OTLIB})
       endif()
     endforeach()
+    target_compile_options(opentelemetry::opentelemetry_trace INTERFACE -Wno-cpp)
   endif()
 endif()