mk: fix static link with glibc < 2.17

mk: fix static link with glibc < 2.17

There is an error when linking static EAL library with an application:

eal_alarm.c:(.text+0xd7): undefined reference to `clock_gettime'
eal_alarm.c:(.text+0x20f): undefined reference to `clock_gettime'
eal_timer.c:(.text+0x108): undefined reference to `clock_gettime'
eal_timer.c:(.text+0x146): undefined reference to `clock_gettime'

The function clock_gettime() is in librt for old glibc.

Fixes: 281948b4753e ("mk: fix missing librt dependencies")
Signed-off-by: Thomas Monjalon <thomas.monjalon@6wind.com>
Tested-by: Yongjie Gu <yongjiex.gu@intel.com>