This avoid conflicts with libc's time.h. Using "-I bionic/libc/include"
is not working anymore, because bionic now exports the same directory
with -isystem, which supersedes the -I version (as stated in GCC doc).
This means the version of time.h in libavutil will now be found first,
via "-I external/ffmpeg/libavutil" (-I dirs are searched first, before
the -isystem ones).
Change-Id: I0d1e4276fbd0ea79d448ca96577528b296a4d462
