signals: fix dependency issues when signals are all disabled

[wangchdo]

Fix build and runtime issues when signals all disabled.

Summary

This PR fixes dependency issues when signals are all disabled

Impact

Fix dependency issues when signals are all disabled, no impact to any existing functions

Testing

ostest passed on rv-virt:smp64

disable all signals

ostest passed log

NuttShell (NSH)
nsh> 
nsh> uname -a
NuttX 0.0.0 02cfe5cf75-dirty Jan 15 2026 16:37:47 risc-v rv-virt
nsh> 
nsh> ostest

(...)

smp_call_test: Test start
smp_call_test: Call cpu 0, nowait
smp_call_test: Call cpu 0, wait
smp_call_test: Call cpu 1, nowait
smp_call_test: Call cpu 1, wait
smp_call_test: Call cpu 2, nowait
smp_call_test: Call cpu 2, wait
smp_call_test: Call cpu 3, nowait
smp_call_test: Call cpu 3, wait
smp_call_test: Call cpu 4, nowait
smp_call_test: Call cpu 4, wait
smp_call_test: Call cpu 5, nowait
smp_call_test: Call cpu 5, wait
smp_call_test: Call cpu 6, nowait
smp_call_test: Call cpu 6, wait
smp_call_test: Call cpu 7, nowait
smp_call_test: Call cpu 7, wait
smp_call_test: Call multi cpu, nowait
smp_call_test: Call in interrupt, wait
smp_call_test: Call multi cpu, wait
smp_call_test: Test success

Final memory usage:
VARIABLE  BEFORE   AFTER
======== ======== ========
arena     1fc2d20  1fc2d20
ordblks         1        7
mxordblk  1fb7c78  1f709c0
uordblks     b0a8    16278
fordblks  1fb7c78  1facaa8
user_main: Exiting
ostest_main: Exiting with status 0
nsh>