CMakeLists.txt: disable cxx support for librdkafka fluent#7741

Fluent-bit is c only, so no need to compile cxx.
This fixes also a compile error in buildroot.

Signed-off-by: Thomas Devoogdt <[email protected]>

Author: ThomasDevoogdt

CMakeLists.txt

@@ -967,6 +967,7 @@ if(FLB_BACKTRACE)
 endif()
 
 if(FLB_IN_KAFKA OR FLB_OUT_KAFKA)
+    FLB_OPTION(RDKAFKA_BUILD_CXX      Off)
     FLB_OPTION(RDKAFKA_BUILD_STATIC    On)
     FLB_OPTION(RDKAFKA_BUILD_EXAMPLES Off)
     FLB_OPTION(RDKAFKA_BUILD_TESTS    Off)