fpu: make softfloat-macros "self-contained"

The macros use the "flags" type and to be consistent if anyone just
needs the macros we should bring in the header we need. There is an
outstanding TODO to audit the use of "flags" and replace with bool at
which point this include could be dropped.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Acked-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>