update(app_actions): add a depraction comment for BPF

Signed-off-by: Andrea Terzolo <andrea.terzolo@polito.it>

userspace/falco/app_actions/open_inspector.cpp

@@ -22,6 +22,7 @@ limitations under the License.
 
 #include "application.h"
 
+/* DEPRECATED: we will remove it in Falco 0.34. */
 #define FALCO_BPF_ENV_VARIABLE "FALCO_BPF_PROBE"
 
 using namespace falco::app;