ocb: fix gcc4.8 compiling problem with __ARM_NEON__

Fixes following problem when compiling mosh with gcc4.8 and
__ARM_NEON__ defined by using -121 instead of 135.

ocb.cc: In function 'block double_block(block)':
ocb.cc:263:56: error: narrowing conversion of '135' from
'int' to '__builtin_neon_qi' inside { } is ill-formed in
C++11 [-Werror=narrowing]

Signed-off-by: Pasi Sjöholm <pasi.sjoholm@jollamobile.com>