src: don't write to possible rodata location

src: don't write to possible rodata location

seen with gcc-16.0.1:
src/libnftables.c: In function 'nft_ctx_add_var':
src/libnftables.c:153:27: warning: initialization discards 'const' qualifier from pointer target type [-Wdiscarded-qualifiers]
153 |         char *separator = strchr(var, '=');

function arg says "const char *", write to this memory is not expected.

Signed-off-by: Florian Westphal <fw@strlen.de>