use WEBP_DSP_INIT_FUNC for Init{GammaTables*,GetCoeffs}

this provides stronger synchronization when pthreads are available as
was done in 'd77bf512 add WEBP_DSP_INIT / WEBP_DSP_INIT_FUNC' for the
other init functions.

Change-Id: I2ffe4e24454d276c2411ece34dca38d23d4756d5
2 files changed