Correct the SSE41 definition

[Genbox]

GCC generates the following definitions:

$ gcc -msse4 -dM -E - < /dev/null | egrep "SSE" | sort
#define __MMX_WITH_SSE__ 1
#define __SSE2_MATH__ 1
#define __SSE2__ 1
#define __SSE3__ 1
#define __SSE4_1__ 1
#define __SSE4_2__ 1
#define __SSE_MATH__ 1
#define __SSE__ 1
#define __SSSE3__ 1

However, several places in the FastFilter source code use __SSE41__. There is a missing underscore. This PR fixes the ifdef to match the ones from GCC. Output is from GCC version 11.4.0.

[lemire]

Thanks.