util: fix compiler warning about deprecated space before _MiB
```
src/util/byte_units.h:13:29: error: identifier '_MiB' preceded by whitespace in a literal operator declaration is deprecated [-Werror,-Wdeprecated-literal-operator]
13 | constexpr size_t operator"" _MiB(unsigned long long mebibytes)
| ~~~~~~~~~~~^~~~
| operator""_MiB
1 error generated.
```
Clang 20.0.0
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