comms/mbpoll: Add missing pkgconfig dependency

CMakeLists.txt calls find_package(PkgConfig) to locate libmodbus, but the
port only declares USES=cmake, so pkgconf is not pulled in as a build
dependency. In a clean jail configure fails with:

Could NOT find PkgConfig (missing: PKG_CONFIG_EXECUTABLE)
Call Stack (most recent call first):
  CMakeLists.txt:59 (find_package)

Reviewed by: mm
Differential Revision: https://reviews.freebsd.org/D59116