cfileio: fix function typedefs to have struct/enum

cfileio: fix function typedefs to have struct/enum

Otherwise building with cfileio.h would result in:

/usr/include/assimp/cfileio.h:61:58: error: unknown type name ‘aiOrigin’
 typedef aiReturn (*aiFileSeek)(C_STRUCT aiFile*, size_t, aiOrigin);
                                                          ^~~~~~~~
/usr/include/assimp/cfileio.h:64:9: error: unknown type name ‘aiFile’
 typedef aiFile* (*aiFileOpenProc)  (C_STRUCT aiFileIO*, const char*, const char*);
         ^~~~~~
/usr/include/assimp/cfileio.h:122:5: error: unknown type name ‘aiFileSeek’
     aiFileSeek SeekProc;
     ^~~~~~~~~~