Update devel/protobuf-c to 1.2.1

protobuf-c (1.2.1)

  [ Robert Edmonds ]
  * Release 1.2.1.

  [ Paolo Borelli ]
  * protoc-c: Generate code that uses the universal zero initializer {0} when
    initializing a oneof union (#187, #205).

protobuf-c (1.2.0)

  [ Robert Edmonds ]
  * Release 1.2.0.

  [ Ilya Lipnitsky ]
  * Implement the "optimize_for = CODE_SIZE" option (#183).

  * Eliminate undefined behavior in zigzag functions (#198).

  * Pack negative enum values correctly (#199).

  [ Peter Leschev ]
  * Fix protobuf_c_message_get_packed_size() on 16-bit systems (#196, #197).

  [ Diego Elio Petten嘆 ]
  * Update link to Autotools Mythbuster to canonical site (#201).

  [ Zex Li ]
  * Skip test suite when cross-compiling (#184).

(wiedi)