test/compress: fix missing header include

usleep() is defined in unistd.h, which is missing from include list
in test_compressdev.c, causing compiler errors on FreeBSD.

Fixes: b06aa643ca ("test/compress: add initial unit tests")
Cc: stable@dpdk.org

Signed-off-by: Bruce Richardson <bruce.richardson@intel.com>

app/test/test_compressdev.c

@@ -4,6 +4,7 @@
 #include <string.h>
 #include <zlib.h>
 #include <math.h>
+#include <unistd.h>
 
 #include <rte_cycles.h>
 #include <rte_malloc.h>