Moved pdkim declaration to satisfy older compilers

As suggested by Dennis Davis to fix an error with gcc 2.95.2
which threw the following error:-

gcc pdkim.c
pdkim.c: In function `pdkim_feed_finish':
pdkim.c:1389: parse error before `*'
pdkim.c:1390: `hdrs' undeclared (first use in this function)
pdkim.c:1390: (Each undeclared identifier is reported only once
pdkim.c:1390: for each function it appears in.)
gmake[2]: *** [pdkim.o] Error 1

See https://lists.exim.org/lurker/message/20120524.094800.89928246.en.html