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>

configure

@@ -14,7 +14,6 @@ check_toolchain()
 : ${CC=gcc}
 echo "AR:=${AR}" >>Config
 echo "CC:=${CC}" >>Config
-echo "PKG_CONFIG:=${PKG_CONFIG}" >>Config
 }
 
 check_atm()
@@ -231,6 +230,7 @@ rm -f $TMPDIR/ipsettest.c $TMPDIR/ipsettest
 }
 
 echo "# Generated config based on" $INCLUDE >Config
+echo "PKG_CONFIG:=${PKG_CONFIG}" >>Config
 check_toolchain
 
 echo "TC schedulers"