Target: fix -Wcast-qual warning

We were unnecessarily stripping the const qualifier on the temporary variable.
Restore the constness to avoid the warning.  NFC.

llvm-svn: 258547

lldb/source/Target/UnixSignals.cpp

@@ -180,7 +180,7 @@ UnixSignals::GetShortName(ConstString name) const
 {
     if (name)
     {
-      char* signame = (char*)(name.AsCString());
+      const char* signame = name.AsCString();
       return ConstString(signame + 3); // Remove "SIG" from name
     }
     return name;