When compiling library files under `3rdparty/`, the directory containing
the `.c` file that is being compiled is not the current directory, so
headers from the `library/` directory are not found. Fix this by
adding `.` to the include path.
This was not detected until now because as of this commit, no 3rdparty
source file requires a header under `library/`.
Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
