modpost: refactor check_sec_ref()

We can replace &elf->sechdrs[i] with &sechdrs[i] to slightly shorten
the code because we already have the local variable 'sechdrs'.

However, defining 'sechdr' instead shortens the code further.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>