[libc-commits] [libc] [libc][signal] clean up usage of sighandler_t (PR #125745)

[Nick Desaulniers via libc-commits]

nickdesaulniers wrote:

failure in presubmit:
```
llvm-project/build/libc/include/signal.h:20:10: fatal error: 'llvm-libc-types/__sighandler_t.h' file not found
   20 | #include "llvm-libc-types/__sighandler_t.h"
      |          ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
```
the `libc.include.signal.__generated_hdr__` target doesn't have a dependency expressed on libc.include.llvm-libc-types.__sighandler_t. Need to add that.

https://github.com/llvm/llvm-project/pull/125745