gitattributes: Set binary attribute for *.gz and *.png files

gitattributes: Set binary attribute for *.gz and *.png files

After commit ("1f35137 Add .gitattributes for line endings") all files
have text attribute set and we need such change to ensure content of
the *.gz and *.png files remains unmodified.

$ git check-attr -a -- *.gz *.png
*.gz: binary: set
*.gz: diff: unset
*.gz: merge: unset
*.gz: text: unset
*.gz: eol: lf
*.png: binary: set
*.png: diff: unset
*.png: merge: unset
*.png: text: unset
*.png: eol: lf

Change-Id: Ie7b1d48761c01c045bdcdae3b0ac66e64af79326
Signed-off-by: Sylwester Nawrocki <s.nawrocki@samsung.com>