Build fails on arm musl host

[petm5]

When attempting to build box86 on Alpine Linux armv7, it fails with the following:

❯ make -j1
Consolidate compiler generated dependencies of target dynarec_arm
[  0%] Building C object CMakeFiles/dynarec_arm.dir/src/dynarec/dynarec_arm_functions.c.o
In file included from /home/peter/box86/src/dynarec/dynarec_arm_functions.c:25:
/home/peter/box86/src/include/signals.h:10:11: error: unknown type name '__sighandler_t'
   10 |           __sighandler_t _sa_handler;
      |           ^~~~~~~~~~~~~~
/home/peter/box86/src/include/signals.h:20:11: error: unknown type name '__sighandler_t'
   20 |           __sighandler_t _sa_handler;
      |           ^~~~~~~~~~~~~~
make[2]: *** [CMakeFiles/dynarec_arm.dir/build.make:104: CMakeFiles/dynarec_arm.dir/src/dynarec/dynarec_arm_functions.c.o] Error 1
make[1]: *** [CMakeFiles/Makefile2:150: CMakeFiles/dynarec_arm.dir/all] Error 2
make: *** [Makefile:166: all] Error 2