math/muparser: Fix pkg-plist

> Checking for items in STAGEDIR missing from pkg-plist

Error: Orphaned: lib/cmake/muparser/muparser-targets-%%CMAKE_BUILD_TYPE%%.cmake
Error: Orphaned: lib/cmake/muparser/muparser-targets.cmake
Error: Orphaned: lib/cmake/muparser/muparserConfig.cmake
Error: Orphaned: lib/cmake/muparser/muparserConfigVersion.cmake

Approved by: portmgr (blanket)