[memutil] Use override keyword, not virtual for overridden method
[memutil] Use override keyword, not virtual for overridden method

I think the virtual keyword was used to denote the fact that this is
overriding a method from SimCtrlExtension, but that's not quite what
"virtual" does.

Fortunately, we're targeting a version >= c++11, so we can use the
"override" keyword to say precisely what we want.

Signed-off-by: Rupert Swarbrick <290093e2ad2515ed2f14d749293c8cf8e02ca274@lowrisc.org>

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