Include serial_reg.h with userspace headers

Include serial_reg.h with userspace headers

As reported by Gustavo de Nardin <gustavodn@mandriva.com.br>, while trying to
compile xosview (http://xosview.sourceforge.net/) with upstream kernel
headers being used you get the following errors:
serialmeter.cc:48:30: error: linux/serial_reg.h: No such file or directory
serialmeter.cc: In member function 'virtual void
SerialMeter::checkResources()':
serialmeter.cc:71: error: 'UART_LSR' was not declared in this scope
serialmeter.cc:71: error: 'UART_MSR' was not declared in this scope
...

Signed-off-by: Herton Ronaldo Krzesinski <herton@mandriva.com.br>
Cc: Gustavo de Nardin <gustavodn@mandriva.com.br>
Cc: David Woodhouse <dwmw2@infradead.org>
Cc: Russell King <rmk@arm.linux.org.uk>
Cc: <stable@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>