tests/ptimer: Remove unnecessary inclusion of libqtest.h

ptimer-test is a unit test, not a qtest, so libqtest.h is not
required here.

Message-Id: <20191218103059.11729-3-thuth@redhat.com>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>