build: add missing virStorageFileResize to libvirt_private.syms

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

src/libvirt_private.syms

@@ -1019,7 +1019,7 @@ virStorageFileIsSharedFS;
 virStorageFileIsSharedFSType;
 virStorageFileProbeFormat;
 virStorageFileProbeFormatFromFD;
-
+virStorageFileResize;
 
 # sysinfo.h
 virSysinfoDefFree;