G201/G202: add checks for injection into sql.Conn methods

We check sql.DB and sql.Tx, but sql.Conn appears to have been missed. It
carries the same issues as DB/Tx in terms of injection.

rules/sql.go

@@ -32,6 +32,12 @@ type sqlStatement struct {
 }
 
 var sqlCallIdents = map[string]map[string]int{
+	"*database/sql.Conn": {
+		"ExecContext":     1,
+		"QueryContext":    1,
+		"QueryRowContext": 1,
+		"PrepareContext":  1,
+	},
 	"*database/sql.DB": {
 		"Exec":            0,
 		"ExecContext":     1,