buildman: Create a requirements.txt file

At this point, buildman requires a few different modules and so we need
a requirements.txt to track what modules are needed.

Cc: Simon Glass <sjg@chromium.org>
Cc: Neha Malcom Francis <n-francis@ti.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Neha Malcom Francis <n-francis@ti.com>
Signed-off-by: Tom Rini <trini@konsulko.com>