Log Message:
volk: update to 2.5.0

Kernels
    volk_32f_stddev_and_mean_32f_x2: implemented Young and Cramer's algorithm
    volk_32fc_accumulator_s32fc: Add new kernel
    volk_16ic_x2_dot_prod_16ic_u_avx2: Fix Typo, was _axv2.
    Remove _mm256_zeroupper() calls
    Enforce consistent function prototypes
    32fc_index_max: Improve speed of AVX2 version
    conv_k7_r2: Disable broken AVX2 code
    improve volk_8i_s32f_convert_32f for ARM NEON
    Calculate cos in AVX512F
    Calculate sin using AVX512F
Build
    Fix python version detection
    cmake: Check that 'distutils' is available
    c11: Remove pre-C11 preprocessor instructions