MDEV-23935: Fix warnings generated during compilation of plugin/auth_pam/testing/pam_mariadb_mtr.c on MacOS

During build server 10.4 on MacOS warnings like the following one
are generated on compiling the file plugin/auth_pam/testing/pam_mariadb_mtr.c

server-10.4/plugin/auth_pam/testing/pam_mariadb_mtr.c:25:22: error:
initializing 'char *' with an expression of type 'const char [23]'
discards qualifiers [-Werror,-Wincompatible-pointer-types-discards-qualifiers]
{ PAM_TEXT_INFO, "Challenge input first." }

The reason of the warnings is that the data member pam_message::msg is declared
as 'char *' on MacOS but initializer of the data member is 'const char *'.

To eliminate warnings the compiler option
  -Wno-incompatible-pointer-types-discards-qualifiers
has been added to the compiler flags used for compiling the file
pam_mariadb_mtr.c.