[libunwind] [libunwind] Move errno.h and signal.h includes under the block where they're needed (PR #78054)

[Louis Dionne via cfe-commits]

ldionne wrote:

CC @ajordanr-google 

By the way, I noticed that you checked the "hide my email address" Github feature, which leads to your commits being authored as `Jordan R AW <103465530+ajordanr-google at users.noreply.github.com>`. Please uncheck that, we strive to make it easy to track who is the author of a commit.

https://github.com/llvm/llvm-project/pull/78054