england-and-wales
It depends
This answer deals with secondary liability as an accomplice. To ensure consistent referencing, I will assume the principal offence (ie. that committed by the rioters) was indictable. This is not a particularly limiting assumption as the statutory law is identical for summary offences (see: Section 44 of the Magistrates' Courts Act 1980). However, if the rioters acted entirely lawfully, then this answer does not apply, as you cannot be an accomplice without there being a principal offender.
Under this assumption, we look to Section 8 of the Accessories and Abettors Act 1861, which states that:
whosoever shall aid, abet, counsel or procure the commission of any indictable offence … shall be tried, indicted and punished as a principal offender.
On this alone, it appears that your question's answer should be yes. However, two mens rea considerations complicate this conclusion.
Firstly, the defendant must have intentionally done the act which assisted (National Coal Board v Gamble). This is straightforwardly satisfied by the facts of your question. Secondly, the defendant must also have had the circumstances of the offence within their contemplation as a material possibility (Johnson v Youden). This is less clear and requires a more careful examination of the distributor's exact knowledge at the time.
To summarise: there is no blanket rule prohibiting someone from distributing face shields to a rioter. However, if any principal offenders are identified, then this act, coupled with the right intention, can lead to the distributor incurring criminal liability as an accomplice.