Fix build errors due to missing header file:

../drivers/pnp/pnpbios/core.c: In function 'pnp_dock_event':
../drivers/pnp/pnpbios/core.c:141:2: error: implicit declaration of function 'call_usermodehelper' [-Werror=implicit-function-declaration]
  value = call_usermodehelper(argv [0], argv, envp, UMH_WAIT_EXEC);
  ^
../drivers/pnp/pnpbios/core.c:141:52: error: 'UMH_WAIT_EXEC' undeclared (first use in this function)
  value = call_usermodehelper(argv [0], argv, envp, UMH_WAIT_EXEC);
                                                    ^
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>