text_renderer: freetype: fix function signature

All other _GetFamily functions have the last argument as const. It fixes
the following warning:

../modules/text_renderer/freetype/platform_fonts.c:925:26: warning: incompatible function pointer types ... [-Wincompatible-function-pointer-types]
    fs->pf_select_family = StaticMap_GetFamily;
                         ^ ~~~~~~~~~~~~~~~~~~~