l3.c doesn't compile on older distros

[gregthelaw]

On 20.04, @gapisback sees:

gcc  -x c   -Ofast -DL3_ENABLED -D_GNU_SOURCE -ggdb3 -Wall -Wfatal-errors -Werror  -I ./include -I ./src/ -I ./use-cases/utils -c src/l3.c -o build/release/obj/src/l3.o
src/l3.c:30:10: fatal error: sys/single_threaded.h: No such file or directory
   30 | #include <sys/single_threaded.h>
      |          ^~~~~~~~~~~~~~~~~~~~~~~
compilation terminated.
make: *** [Makefile:846: build/release/obj/src/l3.o] Error 1

he upgraded to 22.04 and the problem went away; seems like single_threaded.h is a relatively recent addition.