kexec-tools: Call dlclose() from within __xc_interface_close()

This patch is a follow-on to commit 43d3932e "kexec-tools: Perform
run-time linking of libxenctrl.so". This patch addresses feedback
from Daniel Kiper.

In the original patch, the call to dlclose() was omitted, in contrast
to the description in the commit message. This patch inserts the call.
Note that this dynamic linking feature is dependent upon the proper
operation of the RTLD_NODELETE flag to dlopen(), which does work as
advertised on Linux (otherwise the call to dlclose() should be omitted).

Signed-off-by: Eric DeVolder <eric.devolder@oracle.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>
Signed-off-by: Simon Horman <horms@verge.net.au>