core/units: remove space before literal identifier

[tchaikov]

clang 20 complains:

/home/kefu/dev/seastar/include/seastar/core/units.hh:37:29: warning: identifier '_KiB' preceded by whitespace in a literal operator declaration is deprecated [-Wdeprecated-literal-operator]
   37 | constexpr size_t operator"" _KiB(unsigned long long n) { return n << 10; }
      |                  ~~~~~~~~~~~^~~~
      |                  operator""_KiB
/home/kefu/dev/seastar/include/seastar/core/units.hh:38:29: warning: identifier '_MiB' preceded by whitespace in a literal operator declaration is deprecated [-Wdeprecated-literal-operator]
   38 | constexpr size_t operator"" _MiB(unsigned long long n) { return n << 20; }
      |                  ~~~~~~~~~~~^~~~
      |                  operator""_MiB
/home/kefu/dev/seastar/include/seastar/core/units.hh:39:29: warning: identifier '_GiB' preceded by whitespace in a literal operator declaration is deprecated [-Wdeprecated-literal-operator]
   39 | constexpr size_t operator"" _GiB(unsigned long long n) { return n << 30; }
      |                  ~~~~~~~~~~~^~~~
      |                  operator""_GiB
/home/kefu/dev/seastar/include/seastar/core/units.hh:40:29: warning: identifier '_TiB' preceded by whitespace in a literal operator declaration is deprecated [-Wdeprecated-literal-operator]
   40 | constexpr size_t operator"" _TiB(unsigned long long n) { return n << 40; }
      |                  ~~~~~~~~~~~^~~~
      |                  operator""_TiB

because, in
CWG2521, it proposes that compiler should consider

  string operator "" _i18n(const char*, std::size_t); // OK, deprecated

as "OK, deprecated".

and Clang implemented this proposal, as it was accepted by C++23.
Since Seastar supports both C++20 and C++23 standards. let's remove the space between " and _ to be more compliant to the C++23 standard and to silence the warning.