fix build with gcc-15

[heitbaum]

Fixes:

gcc -c tsserve.c -o obj/tsserve.o -march=x86-64-v3 -Wall -pipe  -O2 -fomit-frame-pointer -DNDEBUG -Wall -O2 -g -D_FILE_OFFSET_BITS=64 -I.  -fPIC -DTSTOOLS_VERSION=1.13
tsserve.c: In function 'set_child_exit_handler':
tsserve.c:2988:21: error: assignment to '__sighandler_t' {aka 'void (*)(int)'} from incompatible pointer type 'void (*)(void)' [-Wincompatible-pointer-types]
 2988 |   action.sa_handler = on_child_exit;
      |                     ^

[jacmet]

I ran into the same thing. The issue is that gcc-15 defaults to C23, which changes the meaning of an empty parameter list: https://gcc.gnu.org/gcc-15/porting_to.html#c23-fn-decls-without-parameters