aomdec: remove use of fileno()
aomdec: remove use of fileno()

this requires _POSIX_C_SOURCE >= 1 for glibc, but its only use was with
stdout; use the file number directly

BUG=aomedia:111

Change-Id: Iebfc339c894abbf77b08c7744a4f66b34948b33f

GrokImageCompression/aom,smarter/aom,luctrudeau/aom,luctrudeau/aom,luctrudeau/aom,smarter/aom,smarter/aom,GrokImageCompression/aom,smarter/aom,luctrudeau/aom,GrokImageCompression/aom,GrokImageCompression/aom,smarter/aom,luctrudeau/aom,GrokImageCompression/aom,GrokImageCompression/aom,smarter/aom,luctrudeau/aom