zig fmt: fix extra whitespace in StructInit with multiline strings

[skewb1k]

68d2f68 introduced special handling for StructInit fields containing multiline strings to prevent inserting whitespace after =. However, this logic didn't handle cases without a trailing comma, which resulted in unwanted trailing whitespace.

test {
    _ = .{
        .field =
        \\this is fine
        ,
    };
    _ = .{ .field = 
        \\         ^ trailing whitespace here
    };
}

[skewb1k]

Two source files were affected; rerun zig fmt.