Replace use of deprecated <ext/hash_map>

[donpellegrino]

<ext/hash_map> has been deprecated.

In file included from src/dictionary/PlainDictionary.cpp:34:
In file included from src/dictionary/PlainDictionary.hpp:46:
In file included from /usr/lib/gcc/x86_64-linux-gnu/9/../../../../include/c++/9/ext/hash_map:60:
/usr/lib/gcc/x86_64-linux-gnu/9/../../../../include/c++/9/backward/backward_warning.h:32:2: warning: This file includes
      at least one deprecated or antiquated header which may be removed without further notice at a future date. Please
      use a non-deprecated interface with equivalent functionality instead. For a listing of replacement headers and
      interfaces, consult the file backward_warning.h. To disable this warning use -Wno-deprecated. [-W#warnings]
#warning \
 ^