build: Add 'subdir-objects' to AM_INIT_AUTOMAKE

This will shutup automake and make it stop complaining about the
following subdirectory warnings.

"warning: source file 'common/fft.c' is in a
subdirectory, but option 'subdir-objects' is disabled"

Signed-off-by: Tom Tsou <tom@tsou.cc>
1 file changed