This value is signed in vp9/10

Cleans warning in Android build:
comparison of integers of different signs: 'unsigned int' and 'int'
if (cpi->frames_since_golden == (cpi->current_gf_interval >> 1))
    ~~~~~~~~~~~~~~~~~~~~~~~~ ^   ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Change-Id: Ie137724982f3a46c8c1820548c1960d62a4e96f2