timer.h has been removed from internal.h, and then added back with
3e6088f7 for convenience. This patch removed it again for the
following reasons:

1. Only includes what's necessary is a common and safe strategy.

2. It fixed some build errors on Android:
  a. libavutil/timer.h includes sys/ioctl.h, and ioctl.h includes
     termios.h on Android.
  b. termios.h reserves names prefixed with ‘c_’, ‘V’, ‘I’, ‘O’, and
     ‘TC’; and names prefixed with ‘B’ followed by a digit.
  c. libavcodec uses B0 B1 and so on as variable names a lot. So
     the code failed to build with --enable-linux-perf, or
     --target-os=Linux.
Signed-off-by: Zhao Zhili <zhilizhao@tencent.com>