tests: allow running tests/tst-mmap.cc

Fix a missing include to allow tst-mmap.cc to be compiled on Linux, and
add in a comment the (far from obvious) command line needed to compile it.

Signed-off-by: Nadav Har'El <nyh@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>