lcd4linux

Add drv_generic_serial.c functions to header file

New functions were added in rev1171, but were not added to
drv_generic_serial.h

Fixes Debian buildd [-Wimplicit-function-declaration] warnings.

Signed-off-by: Jonathan McCrohan <jmccrohan@gmail.com>

git-svn-id: https://ssl.bulix.org/svn/lcd4linux/trunk@1188 3ae390bd-cb1e-0410-b409-cd5a39f66f1f