libnftnl: fix tag name

* ${P} doesn't work e.g. with multilib

* fixes:
  git -c gc.autoDetach=false -c core.pager=cat -c safe.bareRepository=all -c clone.defaultRemoteName=origin rev-list -n 1 lib32-libnftnl-1.2.9 failed with exit code 128, output:
  fatal: ambiguous argument \'lib32-libnftnl-1.2.9\': unknown revision or path not in the working tree.

Signed-off-by: Martin Jansa <martin.jansa@gmail.com>
Signed-off-by: Khem Raj <raj.khem@gmail.com>

meta-networking/recipes-filter/libnftnl/libnftnl_1.2.9.bb

@@ -3,7 +3,7 @@ LICENSE = "GPL-2.0-only"
 LIC_FILES_CHKSUM = "file://COPYING;md5=79808397c3355f163c012616125c9e26"
 SECTION = "libs"
 
-SRC_URI = "git://git.netfilter.org/libnftnl;branch=master;tag=${P} \
+SRC_URI = "git://git.netfilter.org/libnftnl;branch=master;tag=${BP} \
            file://0001-configure.ac-Add-serial-tests.patch \
            file://run-ptest \
            "