[gdb] Print user for maint info psymtabs

The type struct partial_symtab contains two fields (disregarding field next)
that express relations with other symtabs: user and dependencies.

When using "maint print psymbols", we see both the dependencies and the user
fields:
...
Partial symtab for source file  (object 0x35ef270)
  ...
  Depends on 0 other partial symtabs.
  Shared partial symtab with user 0x35d5f40
...

But with "maint info psymtabs", we only see dependencies:
...
  { psymtab  ((struct partial_symtab *) 0x35ef270)
    ...
    dependencies (none)
  }
...

Add printing of the user field for "maint info psymtabs", such that we have:
...
   { psymtab  ((struct partial_symtab *) 0x35ef270)
     ...
+    user hello.c ((struct partial_symtab *) 0x35d5f40)
     dependencies (none)
   }
...

Tested on x86_64-linux.

gdb/ChangeLog:

2020-03-24  Tom de Vries  <tdevries@suse.de>

	* psymtab.c (maintenance_info_psymtabs): Print user field.