efi_loader: add missing const qualifier

This commit fixes the following compilation warning
of boottime->install_configuration_table() function.

lib/efi_selftest/efi_selftest_tcg2.c:475:46:
warning: passing argument 1 of ‘boottime->install_configuration_table’
discards ‘const’ qualifier from pointer target type [-Wdiscarded-qualifiers]
  ret = boottime->install_configuration_table(&smbios_guid, dmi);

Signed-off-by: Masahisa Kojima <masahisa.kojima@linaro.org>
Reviewed-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>