Follow-up to 99ceb03a1cfcf35968cab589ea188a8c406cda52

In file included from src/video_core/dma_pusher.cpp:5:
src/./common/cityhash.h:69:47: error: 'size_t' has not been declared
   69 | [[nodiscard]] u64 CityHash64(const char* buf, size_t len);
      |                                               ^~~~~~
src/./common/cityhash.h:73:55: error: 'size_t' has not been declared
   73 | [[nodiscard]] u64 CityHash64WithSeed(const char* buf, size_t len, u64 seed);
      |                                                       ^~~~~~
src/./common/cityhash.h:77:56: error: 'size_t' has not been declared
   77 | [[nodiscard]] u64 CityHash64WithSeeds(const char* buf, size_t len, u64 seed0, u64 seed1);
      |                                                        ^~~~~~
src/./common/cityhash.h:80:47: error: 'size_t' has not been declared
   80 | [[nodiscard]] u128 CityHash128(const char* s, size_t len);
      |                                               ^~~~~~
src/./common/cityhash.h:84:55: error: 'size_t' has not been declared
   84 | [[nodiscard]] u128 CityHash128WithSeed(const char* s, size_t len, u128 seed);
      |                                                       ^~~~~~

Allow sharing return types with the rest of the code base. For example,
we use 'u128 = std::array<u64, 2>', meanwhile Google's code uses
'uint128 = std::pair<u64, u64>'.

While we are at it, use size_t instead of std::size_t.

Now that clang-format makes [[nodiscard]] attributes format sensibly, we
can apply them to several functions within the common library to allow
the compiler to complain about any misuses of the functions.