fix is_reentrant for internal vyper functions

[0xalpharush]

is_reentrant was returning True for internal functions even if all entry points had the nonreentrant decorator, causing FPs on examples like:

balances: HashMap[address, uint256]

@external
@nonreentrant("lock")
def withdraw_locked():
    self.withdraw()

@internal
def withdraw():
    raw_call(msg.sender, b"", value= self.balances[msg.sender])
    self.balances[msg.sender] = 0