Moved pdkim declaration to satisfy older compilers

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