package/runc: use BR_PATH to find pkg-config

package/runc: use BR_PATH to find pkg-config

When BR2_PACKAGE_LIBSECCOMP is set, go will try to find libseccomp
library using pkg-config. But pkg-config is not in the PATH when go
is called during the runc build.

Set the correct PATH in RUNC_MAKE_ENV.

Fixes:
http://autobuild.buildroot.net/results/fea/fea069fec671b32bffd3d133898d71d45abab552

Signed-off-by: Romain Naour <romain.naour@gmail.com>
Cc: Christian Stewart <christian@paral.in>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>