Add a suffix to all the template files

To help with other build system not using meson and to auto generate their build rules, it will be nice to have a suffix to all the template files.

libdav1d_tmpl_sources = files(
    'ipred.c',
    'itx.c',
    'ipred_prepare.c',
    'lf_apply.c',
    'loopfilter.c',
    'mc.c',
    'cdef_apply.c',
    'cdef.c',
    'lr_apply.c',
    'looprestoration.c',
    'recon.c'
)