Clarify documentation for private symbols

The instruction "See Makefile.am" in libvirt.private_syms
always makes me think that this file is autogenerated
and should not be touched manually. This patch spares
every reader of libvirt.private_syms the hassle of
reading Makefile.am before augmenting libvirt.private_syms.
Signed-off-by: NWolfgang Mauerer <wolfgang.mauerer@siemens.com>
Signed-off-by: NJan Kiszka <jan.kiszka@siemens.com>

src/libvirt_private.syms

 #
-# General private symbols. See Makefile.am.
+# General private symbols. Add symbols here, and see Makefile.am for
+# more details.
 #