Drop Init_pg_query from exported symbol map for macOS builds

`Init_pg_query` isn't actually exported, but `_Init_pg_query` is:

```
% nm pg_query_ruby.o | grep Init
0000000000000000 T _Init_pg_query
0000000000003b40 b _Init_pg_query.rb_intern_id_cache
```

A Ruby compiler compiled with XCode 14 without the flag
`-undefined,dynamic_lookup` will see this error:

```
linking shared-object pg_query/pg_query.bundle
Undefined symbols for architecture arm64:
  "Init_pg_query", referenced from:
     -exported_symbol[s_list] command line option
     (maybe you meant: _Init_pg_query)
ld: symbol(s) not found for architecture arm64
clang: error: linker command failed with exit code 1 (use -v to see invocation)
make: *** [pg_query.bundle] Error 1
```

Closes #255

ext/pg_query/extconf.rb

@@ -11,11 +11,12 @@
 
 $INCFLAGS = "-I#{File.join(__dir__, 'include')} " + $INCFLAGS
 
-SYMFILE = File.join(__dir__, 'pg_query_ruby.sym')
 if RUBY_PLATFORM =~ /darwin/
-  $DLDFLAGS << " -Wl,-exported_symbols_list #{SYMFILE}" unless defined?(::Rubinius)
+  symfile = File.join(__dir__, 'pg_query_ruby_darwin.sym')
+  $DLDFLAGS << " -Wl,-exported_symbols_list #{symfile}" unless defined?(::Rubinius)
 else
-  $DLDFLAGS << " -Wl,--retain-symbols-file=#{SYMFILE}"
+  symfile = File.join(__dir__, 'pg_query_ruby.sym')
+  $DLDFLAGS << " -Wl,--retain-symbols-file=#{symfile}"
 end
 
 create_makefile 'pg_query/pg_query'

ext/pg_query/pg_query_ruby_darwin.sym

@@ -0,0 +1 @@
+_Init_pg_query