gtkmathview: fix build error

When building, I was seeing the following error which this patch fixes:

	../../../src/frontend/common/TemplateElementIterator.hh:46:18: error:
	'valid' was not declared in this scope, and no declarations were found by
	argument-dependent lookup at the point of instantiation [-fpermissive]

Upstream-Status: Submitted [padovani .at. di .dot. unito .dot. it]
Signed-off-by: Trevor Woerner <trevor.woerner@linaro.org>
Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>