intel: Fix build failure in test_decode.c

Hi list

The recently released libdrm 2.4.37 does not compile the Intel part:

test_decode.c: In function 'compare_batch':
test_decode.c:107: error: implicit declaration of function 'open_memstream'

PS: Please CC me.

Signed-off-by: Lauri Kasanen <cand@gmx.com>
Signed-off-by: Ben Widawsky <ben@bwidawsk.net>