BUILD: fix build warning when not using 51degrees

An ifdef was missing to avoid declaring these variables :

src/haproxy.c: In function 'deinit':
src/haproxy.c:1253:47: warning: unused variable '_51d_prop_nameb' [-Wunused-variable]
src/haproxy.c:1253:30: warning: unused variable '_51d_prop_name' [-Wunused-variable]
1 file changed