mkerr: Fix string literal conversion

mkerr: Fix string literal conversion

This fixes a compiler warning on clang-1205.0.22.9 when compiling the
generated code as C++11:

ISO C++11 does not allow conversion from string literal to 'char *'
[-Wwritable-strings]

CLA: trivial

Signed-off-by: Sven Schwermer <sven.schwermer@disruptive-technologies.com>
Reviewed-by: Paul Dale <pauli@openssl.org>
Reviewed-by: Richard Levitte <levitte@openssl.org>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/15495)