scripts: mailmapper: SPDX license identifier

If the SPDX license identifier is in the first line the shell does not
recognize which interpreter shall be used to execute the script.

Cf. https://www.kernel.org/doc/html/v4.16/process/license-rules.html


for scripts which require the '#!PATH_TO_INTERPRETER' in the first line
(...) the SPDX identifier goes into the second line.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>