util: add missing export

Commit 1fe2927a forgot to export a symbol.

* src/libvirt_private.syms (virHexToBin): Add.
* src/.gitignore: Ignore temporary file.

src/.gitignore

@@ -14,4 +14,5 @@ libvirt.syms
 libvirt_qemu.def
 *.i
 *.s
+remote_protocol-structs-t
 virt-aa-helper

src/libvirt_private.syms

@@ -668,6 +668,7 @@ virFileResolveLink;
 saferead;
 safewrite;
 safezero;
+virHexToBin;
 virMacAddrCompare;
 virEnumFromString;
 virEnumToString;