rsyslog: inherit pkgconfig

* without pkgconfig it's failing to expand PKG_CHECK_MODULES macro:
  | rsyslog-5.8.0/configure: line 16111: syntax error near unexpected token `GNUTLS,'
  | rsyslog-5.8.0/configure: line 16111: `    PKG_CHECK_MODULES(GNUTLS, gnutls >= 1.4.0)'

Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>