pkg-download: fix indentation for SOURCE_CHECK_* macros

Some of the SOURCE_CHECK_* macros are using a non-standard two-spaces
indentation. This commit switches them to use a single tab based
indentation, like in the rest of Buildroot.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Acked-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Tested-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
[with git show -w]
Acked-by: "Yann E. MORIN" <yann.morin.1998@free.fr>