Make crypto/constant_time inline functions static

Fixes errors like the following when using extra warning and error flags with
clang:

In file included from s3_cbc.c:56:
./../crypto/constant_time_locl.h:122:9: error: static function 'constant_time_msb' is used in an inline function with external linkage
      [-Werror,-Wstatic-in-inline]
        return constant_time_msb(lt);
               ^
./../crypto/constant_time_locl.h:84:1: note: use 'static' to give inline function 'constant_time_lt' internal linkage
inline unsigned int constant_time_lt(unsigned int a, unsigned int b);
^
static
./../crypto/constant_time_locl.h:79:28: note: 'constant_time_msb' declared here
static inline unsigned int constant_time_msb(unsigned int a);
                           ^
Signed-off-by: Mike Bland <mbland@acm.org>