Remove obsolete virGetPMCapabilities sym from private symbols file

Fix the build on Mingw32 by removing the now obsolete
virGetPMCapabilities symbol from the private exports file

* src/libvirt_private.syms: Remove virGetPMCapabilities

src/libvirt_private.syms

@@ -1103,7 +1103,6 @@ virFormatMacAddr;
 virGenerateMacAddr;
 virGetGroupID;
 virGetHostname;
-virGetPMCapabilities;
 virGetUserDirectory;
 virGetUserID;
 virGetUserName;