fix: export vtable/typeinfo symbols of zvec classes in Linux version script

The version script only exported zvec::* function/data symbols. vtable,
typeinfo, VTT and guard variable symbols demangle to 'vtable for zvec::...'
etc., which the zvec::* pattern does not match, so they were localized by
'local: *'. Consumers linking libzvec.so then failed with
'undefined reference to vtable for zvec::core_interface::HNSWIndexParam'.

Add the mangled-name patterns (_ZTVN4zvec*, _ZTIN4zvec*, _ZTSN4zvec*,
_ZTTN4zvec*, _ZGVN4zvec*), mirroring what exported_symbols_cpp.txt already
does on macOS.

Author: chinaux

src/exported_symbols_cpp.lds

@@ -5,6 +5,16 @@
       zvec::*;
     };
     zvec_*;
+    /* vtable / typeinfo / VTT / guard variables of zvec:: classes.
+       Their demangled names start with "vtable for" etc., so the
+       zvec::* patterns above do not match them; without these the
+       consumers fail with "undefined reference to vtable". Mirrors
+       __ZTVN4zvec* etc. in exported_symbols_cpp.txt on macOS. */
+    _ZTVN4zvec*;
+    _ZTIN4zvec*;
+    _ZTSN4zvec*;
+    _ZTTN4zvec*;
+    _ZGVN4zvec*;
   local:
     *;
 };