fix bug when ffmepg not in stardard directory

[GerenLiu]

when ffmpeg installed in customer directory，with options --prefix=/customer_path , this cmake jobs can not found ffmepg in correct, because ${PC_LIB${_component}_INCLUDEDIR} is not defined

  find_path(${_component}_INCLUDE_DIRS ${_header}
    HINTS
      ${PC_LIB${_component}_INCLUDEDIR}
      ${PC_LIB${_component}_INCLUDE_DIRS}
    PATH_SUFFIXES
      ffmpeg
  )

[snikulov]

@GerenLiu Thank you! Landed here bae0b71