libxl: Correct license header on libxl_flask.c to be LGPL

libxl: Correct license header on libxl_flask.c to be LGPL

libxl is intended to be an LGPL 2.1 licensed library, however this
file inadvertently got given a GPL header.

The following people have touched this file, although all but Machon's
contributions are trivial and/or mechanical an Ack from each would be
unambiguous:

$ git log --format='%an <%aE>' tools/libxl/libxl_flask.c | sort -u
Ian Campbell <ian.campbell@citrix.com>
Ian Jackson <ian.jackson@eu.citrix.com>
Machon Gregory <mbgrego@tycho.ncsc.mil>
Wei Liu <liuw@liuw.name>
$

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Cc: Ian Jackson <ian.jackson@eu.citrix.com>
Cc: Machon Gregory <mbgrego@tycho.ncsc.mil>
Cc: Wei Liu <liuw@liuw.name>
Cc: Daniel De Graaf <dgdegra@tycho.nsa.gov>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>
Acked-by: Wei Liu <liuw@liuw.name>
Acked-by: M. Gregory <mbgrego@tycho.ncsc.mil>