Remove DEBUG_STACKFLAGS to bypass compiler error

Remove DEBUG_STACKFLAGS to bypass compiler error

'Support -fsanitize=address with --enable-asan' (fed9035) removed
DEBUG_STACKFLAGS="-fstack-check" from zfs-build.m4 in master.
However, that's too heavyweight a patch to merge in to the 0.7.x branch,
so just take the one-liner we need to get around a compiler error
on Fedora 28:

$ ./configure --enable-debug --enable-debuginfo && make pkg-utils
  CC       gethrtime.lo
cc1: error: '-fstack-check=' and '-fstack-clash_protection' are mutually
exclusive.  Disabling '-fstack-check=' [-Werror]

Signed-off-by: Tony Hutter <hutter2@llnl.gov>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Requires-spl: #701