cgi-io: Find and search for libubus.h

Fixes build errors with external toolchains:

[ 33%] Building C object CMakeFiles/cgi-io.dir/main.c.o
/home/florian/dev/openwrt/trunk/build_dir/target-mipsel-unknown-linux-gnu_glibc/cgi-io/main.c:30:21:
fatal error: libubus.h: No such file or directory
 #include <libubus.h>
                     ^
compilation terminated.

Signed-off-by: Florian Fainelli <[email protected]>

treewide: replace my defunct email address

Signed-off-by: Jo-Philipp Wich <[email protected]>

cgi-io: add a small helper cgi that can be used by RPCD based UIs

Signed-off-by: John Crispin <[email protected]>