Fix comment for 'gdb_dlopen'

The 'gdb_dlopen' function doesn't return NULL if the shlib load
fails; it actually throws an error.  This patch updates the comment
to reflect this.

gdbsupport/ChangeLog:
2020-02-28  Sergio Durigan Junior  <sergiodj@redhat.com>

	* gdb-dlfcn.h (gdb_dlopen): Update comment.