staging: speakup: Prefix set_mask_bits() symbol

This is part of commit ca2beaf84d96 ('staging: speakup: Prefix
externally-visible symbols') upstream.  It is required as preparation
for commit 00a1a053ebe5 ('ext4: atomically set inode->i_flags in
ext4_set_inode_flags()').

Signed-off-by: Ben Hutchings <ben@decadent.org.uk>