Buildroot forces procps-ng to install binaries in the /bin insted of /usr/bin.
As a side effect libprocps.pc was installed at /lib/pkgconfig instead of
/usr/lib/pkgconfig. Which directory is not included into final image.

Added patch taken from buildroot's repo:
0e07a5ede package/procps-ng: install libprocps.pc in staging/usr/lib/pkgconfig/
Signed-off-by: Adam Wujek <adam.wujek@cern.ch>