build: fix missing pthread dependency for syslibs pc

SYS_LIBS var is used to generate Lib.private section of
spdk_syslibs.pc

lack of pthread can result in linking issue when spdk's *.pc are used

please note that usage of -lpthread and -pthread is not consisent
within system wide pc files

Signed-off-by: Jacek Kalwas <jacek.kalwas@intel.com>
Change-Id: I2dd327371d5621ea12f1e2f7abdfe6b7aa6602eb
Reviewed-on: https://review.spdk.io/gerrit/c/spdk/spdk/+/7253


Tested-by: SPDK CI Jenkins <sys_sgci@intel.com>
Reviewed-by: Maciej Szulik <maciej.szulik@intel.com>
Reviewed-by: Jim Harris <james.r.harris@intel.com>
Reviewed-by: Ben Walker <benjamin.walker@intel.com>
Community-CI: Mellanox Build Bot