comms/mbpoll: Add missing pkgconfig dependency

[olgeni]

Summary

mbpoll's CMakeLists.txt calls find_package(PkgConfig) at line 59 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)

The port builds on developer machines only because pkgconf happens to be
installed already. Add pkgconfig to USES.

No PORTREVISION bump: the resulting package is unchanged, the port simply
did not build in a clean environment.

Test Plan

poudriere testport, 14.4 amd64, nox11 set: build succeeds, configure now
reports "Found PkgConfig: /usr/local/bin/pkgconf", check-plist clean.

portlint -AC and portclippy are clean.

[mm]

Please commit