travis: add AddressSanitizer build rules

The AddressSanitizer sanitizer will be used only when compiling with
clang and performing testing as root, so that the sanitizer has maximal
coverage.

Signed-off-by: Sami Kerola <kerolasa@iki.fi>
blkdiscard: include missing header

Commit 700031ade7cbcdcecf1205f98f50ed0da57c7493 missed an include.

sys-utils/blkdiscard.c:182:2: warning: implicit declaration of function
'gettime_monotonic' is invalid in C99 [-Wimplicit-function-declaration]

Signed-off-by: Sami Kerola <kerolasa@iki.fi>