activation_code: fix missing const for item_end
Fix for:
…/src/ipa/libipa/activation_code.c: In function ‘ipa_activation_code_parse’:
…/src/ipa/libipa/activation_code.c:62:26: error: assignment discards ‘const’ qualifier from pointer target type [-Werror=discarded-qualifiers]
62 | item_end = strchr(item + 1, '$');
| ^
Change-Id: Idcdd06f57da0e7c05ea184c56cadfc6de85514e6
