meson: build: Generate the error.cpp/hpp files

Tested: The meson-generated error.hpp file in the build subdir was
        identical to the autotools one:
$ diff ./xyz/openbmc_project/Software/Image/error.hpp ./build/xyz/openbmc_project/Software/Image/error.hpp
$

Change-Id: I26ef286e8f13c8d72f6a4c83f272bbab35454af0
Signed-off-by: Adriana Kobylak <anoo@us.ibm.com>
2 files changed