fix build on gcc-10 (-fno-common)

[trofi]

gcc-10 changed the default from -fcommon to fno-common: https://gcc.gnu.org/PR85678

As a result build fails as:

ld: src/util.c.1.o:/build/pflask/build/../src/printf.h:42: multiple definition of
  `use_syslog'; src/cgroup.c.1.o:/build/pflask/build/../src/printf.h:42: first defined here