regress/Makefile.in: Declare <client>_c.o as .SECONDARY

regress/Makefile.in: Declare <client>_c.o as .SECONDARY

Otherwise you can get this:

 regress$ make adnstest_record -j2
 gcc -g -O2 -Wall -Wmissing-prototypes -Wwrite-strings -Wstrict-prototypes -Wcast-qual -Wpointer-arith -Wno-pointer-sign  -I../src -I. -I./../src  -DADNS_REGRESS_TEST -I. -c -g -o addrfam_d.o ../src/addrfam.c
 gcc    adnstest_c.o hrecord.o hcommon.o types_d.o event_d.o query_d.o reply_d.o general_d.o setup_d.o transmit_d.o parse_d.o poll_d.o check_d.o addrfam_d.o  -o adnstest_record
 gcc: error: adnstest_c.o: No such file or directory
 make: *** [adnstest_record] Error 1
 regress$

Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>