Add ability for rulesets to access falco engine state

Some rulesets may need information which is held by the falco_engine
that created this ruleset. So define a set of functions in a struct
and have setters/getters for those functions in the base class.

Derived classes can use the struct's functions to obtain the falco
engine information.

The only function so far is to obtain the filter_ruleset for a given
event source.

Signed-off-by: Mark Stemm <mark.stemm@gmail.com>

userspace/engine/CMakeLists.txt

@@ -16,6 +16,7 @@ add_library(falco_engine STATIC
     falco_engine.cpp
     falco_load_result.cpp
     falco_utils.cpp
+    filter_ruleset.cpp
     evttype_index_ruleset.cpp
     formats.cpp
     filter_details_resolver.cpp