pmci: mctp: Fix No SOURCES CMake warning

The commit 1e0af58b ("pmci: Move MCTP into the PMCI subsys") changes
'mctp' directory adding to unconditionally, which will cause:

CMake Warning at ./zephyrproject/zephyr/CMakeLists.txt:1022 (message):
  No SOURCES given to Zephyr library: subsys__pmci__mctp

  Excluding target from build.

Signed-off-by: Haiyue Wang <haiyuewa@163.com>