Staging: fbtft: add declaration of non-static functions

This patch fixes these sparse warnings:
drivers/staging/fbtft/fbtft-sysfs.c:23:5: warning: symbol 'fbtft_gamma_parse_str' was not declared. Should it be static?
drivers/staging/fbtft/fbtft-sysfs.c:153:6: warning: symbol 'fbtft_expand_debug_value' was not declared. Should it be static?
drivers/staging/fbtft/fbtft-sysfs.c:209:6: warning: symbol 'fbtft_sysfs_init' was not declared. Should it be static?
drivers/staging/fbtft/fbtft-sysfs.c:216:6: warning: symbol 'fbtft_sysfs_exit' was not declared. Should it be static?

Signed-off-by: Peter Poklop <peter.poklop@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>