libqrencode: select libpng only when tools are selected

As stated on the libqrencode web site, PNG (or SDL) is only
needed for the test and utility programs; the library itself
has no dependencies.

While we are here, remove spaces in the ifeq clause, to match
the examples in the buildroot manual.

Signed-off-by: Danomi Manchego <danomimanchego123@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>