build: unbreak linakge of m_xt.so

build: unbreak linakge of m_xt.so

Commit v3.7.0~10 caused the variable new PKG_CONFIG variable never
to be present at the time of calling make, leading to tc/m_xt.so
not linked with -lxtables (result from pkg-config xtables --libs),
that in turn leading to

tc: symbol lookup error: /usr/lib64/tc//m_xt.so: undefined symbol:
xtables_init_all

Fixing that.

Signed-off-by: Jan Engelhardt <jengelh@inai.de>