soc: esp32: Enable building with newer ESP-IDF

ESP-IDF commit cb31222e added the dependency on a file named
"sdkconfig.h", which is equivalent to "autoconf.h" generated by kbuild
used in Zephyr.  It does not depend on anything from that file, though,
so just provide an empty file to keep the compiler from complaining.

Signed-off-by: Leandro Pereira <leandro.pereira@intel.com>