GH-554: don't init PIA endpoint matchers if there's only file-magic

The logic for initializing PIA endpoint matchers was previously
skipped if "there's no global rule matcher", and that's only true
when no signature files get loaded.

But when using `zeek -b`, some file-magic signatures still get loaded
by default, so the PIA endpoint matchers still get initialized even
though they don't need to be -- file-magic patterns play no part
in PIA.

For typical use-cases (not using the `-b` flag), this change won't
help any, but we do at least use `-b` often within the test suite.

src/RuleMatcher.h

@@ -286,6 +286,8 @@ public:
 	void AddRule(Rule* rule);
 	void SetParseError()		{ parse_error = true; }
 
+	bool HasNonFileMagicRule() const	{ return has_non_file_magic_rule; }
+
 	// Interface to for getting some statistics
 	struct Stats {
 		unsigned int matchers;	// # distinct RE matchers
@@ -356,6 +358,7 @@ private:
 	                                   const AcceptingMatchSet& ams);
 
 	int RE_level;
+	bool has_non_file_magic_rule;
 	bool parse_error;
 	RuleHdrTest* root;
 	rule_list rules;