util: fix build with CFLAGS="-g"

[wfrisch]

The build process fails with debug symbols enabled, at least on my system.
CFLAGS="-g" make V=1:

cc -g -DSMC_TOOLS_RELEASE=1.8.3 -I/usr/include/libnl3  smcd.o infod.o ueidd.o seidd.o devd.o linkgroupd.o statsd.o libnetlink.o util.o  -lnl-genl-3 -lnl-3 -lm -o smcd
[...]
/home/user/smc-tools/dev.c:151:(.text+0x246): undefined reference to `is_str_empty'

It appears the root cause is that CFLAGS=-g influences the compiler's inlining decisions, leading to a situation where the is_str_empty function is not inlined, and the linker is searching for an out-of-line definition that isn’t present.

The most straightforward fix is to declare this function static inline.

[JanKarcher]

Hi Wolfgang,
thanks for pointing this out.
We are going to include this fix in the next version. Can you send your s-o-b to include in your commit?
Thanks
Jan

[wfrisch]

Hi Jan,

done. I just updated my branch with a more detailed commit message.

All the best,
Wolfgang