build: add missing virCommandAddCap to libvirt_private.syms

This was forgotten when the function was initially written (not
noticed because it wasn't used at the time). It's required for proper
compilation with modules enabled after applying the recent rawio
patches.

src/libvirt_private.syms

@@ -117,6 +117,7 @@ virCommandAddEnvPair;
 virCommandAddEnvPass;
 virCommandAddEnvPassCommon;
 virCommandAddEnvString;
+virCommandAllowCap;
 virCommandClearCaps;
 virCommandDaemonize;
 virCommandExec;