Add generated libvirt_admin.syms into .gitignore

Commit a2c5d16a switched to generating
libvirt_admin.syms, but forgot to add the generated file into
.gitignore, hence causing tree pollution post-build.
Signed-off-by: NMartin Kletzander <mkletzan@redhat.com>

.gitignore

@@ -124,6 +124,7 @@
 /src/libvirt_access_lxc.xml
 /src/libvirt_access_qemu.syms
 /src/libvirt_access_qemu.xml
+/src/libvirt_admin.syms
 /src/libvirt_*.stp
 /src/libvirt_*helper
 /src/libvirt_*probes.h