Rename errno.h and stdio.h to swiftsyntax_errno.h and swiftsyntax_stdio.h

In some build scenarios compilers get confused if a file named `errno.h` includes `errno.h`, even if it’s intended to be from a different directory. Renaming the files in swift-syntax works around that issue.

Author: ahoppen

Sources/_SwiftSyntaxCShims/include/swiftsyntax/AtomicBool.h renamed to Sources/_SwiftSyntaxCShims/include/AtomicBool.h

@@ -1,7 +1,7 @@
 module _SwiftSyntaxCShims {
-  header "swiftsyntax/_includes.h"
-  header "swiftsyntax/AtomicBool.h"
-  header "swiftsyntax/errno.h"
-  header "swiftsyntax/stdio.h"
+  header "_includes.h"
+  header "AtomicBool.h"
+  header "swiftsyntax_errno.h"
+  header "swiftsyntax_stdio.h"
   export *
 }