Fix explicit constructor with default arguments check bug

Before this fix, the code:
```
class A {
    A(int, int x=3){
        x;
    }
};
```
Was considered OK.
But explicit keyword is still needed

I'm still new to open-source contributions, so I will gladly take advice.

lib/checkclass.cpp

@@ -273,7 +273,7 @@ void CheckClass::checkExplicitConstructors()
                 continue;
 
             if (!func.isExplicit() &&
-                func.argCount() == 1 &&
+                func.minArgCount() == 1 &&
                 func.type != Function::eCopyConstructor &&
                 func.type != Function::eMoveConstructor) {
                 noExplicitConstructorError(func.tokenDef, scope->className, scope->type == Scope::eStruct);

test/testclass.cpp

@@ -430,6 +430,11 @@ private:
                                   "    B(const B&) {}\n"
                                   "};");
         ASSERT_EQUALS("", errout.str());
+
+        checkExplicitConstructors("struct A{"
+                                  "    A(int, int y=2) {}"
+                                  "};");
+        ASSERT_EQUALS("[test.cpp:1]: (style) Struct 'A' has a constructor with 1 argument that is not explicit.\n", errout.str());
     }
 
     void checkDuplInheritedMembers(const char code[]) {