hal: Add empty rtapi_app_exit to enum component

rtapi_app.h unconditionally declares EXPORT_SYMBOL for both
rtapi_app_main and rtapi_app_exit, so every module is expected to
provide both entry points. enum.c only implemented rtapi_app_main,
which slipped past GNU ld but trips lld 17+'s default
--no-undefined-version (LLVM D135402) when the generated version
script references the missing symbol:

    ld.lld: error: version script assignment of 'global' to symbol
    'rtapi_app_exit' failed: symbol not defined

Provide an empty rtapi_app_exit so enum.c satisfies the contract.
Builds clean with clang 19 / ld.lld 19 for a uspace configuration.

Fixes #3191.

Author: Luca Toniolo

src/hal/components/enum.c

@@ -184,6 +184,8 @@ int rtapi_app_main(void){
 
 }
 
+void rtapi_app_exit(void) {}
+
 static void decode(void *v_inst, long period){
     (void)period;
     int i;