Use PERMISSIONS keyword instead of FILE_PERMISSIONS

Fixes #6

postinstall.cmake.in

 		ERROR_QUIET
 		OUTPUT_STRIP_TRAILING_WHITESPACE
 	)
-file(INSTALL FILES "@CMAKE_CURRENT_BINARY_DIR@/${SECRET_FILE}" DESTINATION "${SYSCONF_DIR}" RENAME "${SECRET_FILE}" FILE_PERMISSIONS OWNER_READ OWNER_WRITE)
+file(INSTALL FILES "@CMAKE_CURRENT_BINARY_DIR@/${SECRET_FILE}" DESTINATION "${SYSCONF_DIR}" RENAME "${SECRET_FILE}" PERMISSIONS OWNER_READ OWNER_WRITE)
 file(REMOVE "@CMAKE_CURRENT_BINARY_DIR@/${SECRET_FILE}")
 endif()