BUILD/MINOR: systemd: fix compiler warning about unused result

There is a compiler warning after commit 1b6e75fa84 ("MEDIUM: haproxy-
systemd-wrapper: Use haproxy in same directory"):

src/haproxy-systemd-wrapper.c: In function ‘locate_haproxy’:
src/haproxy-systemd-wrapper.c:28:10: warning: ignoring return value of ‘readlink’, declared with attribute warn_unused_result [-Wunused-result]

Fix the compiler warning by checking the return value of readlink().