bsdgrep(1): Don't initialize fts_flags twice

Admittedly, this is a clang-scan complaint... but it wasn't wrong. fts_flags
is initialized by all cases in the switch(), which should be fairly obvious.
Annotate this anyways.

usr.bin/grep/util.c

@@ -148,9 +148,9 @@ grep_tree(char **argv)
 	bool matched, ok;
 	const char *wd[] = { ".", NULL };
 
-	fts_flags = 0;
 	matched = false;
 
+	/* This switch effectively initializes 'fts_flags' */
 	switch(linkbehave) {
 	case LINK_EXPLICIT:
 		fts_flags = FTS_COMFOLLOW;