mk: fix link to not combined libraries

mk: fix link to not combined libraries

Commit 944088c2abbe ("fix link to combined library") introduced
a check with a wrong configuration option name.
So link is broken in the case combined library is not enabled.

main.o: In function `rte_pktmbuf_free':
main.c:(.text+0x9c): undefined reference to `per_lcore__lcore_id'

Signed-off-by: Yong Liu <yong.liu@intel.com>
Acked-by: Bruce Richardson <bruce.richardson@intel.com>
[Thomas: fix option name in comments]
Acked-by: Thomas Monjalon <thomas.monjalon@6wind.com>