Include needed unordered_map header

Compiling on FreeBSD 13.2 with the default clang version 14.0.5 and top level
configure options --with-python=/usr/local/bin/python3.9 gives this error:

  CXX    ada-exp.o
./../binutils-gdb/gdb/ada-exp.y:100:8: error: no template named 'unordered_map' in namespace 'std'
  std::unordered_map<std::string, std::vector<ada_index_var_operation *>>
  ~~~~~^
1 error generated.

This change fixes it.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=31918
Approved-By: Tom Tromey <tom@tromey.com>