Fix stubs for non-glibc (#59)

`caml/signals.h` is not public API; it does not include `caml/config.h`
and so it will not include `signal.h` because it does not know about
`POSIX_SIGNAL`. This issue is visible on musl as "unknown type
sigset_t".

Signed-off-by: Etienne Millon <me@emillon.org>

src/spawn_stubs.c

@@ -15,17 +15,15 @@
 #endif
 #endif
 
-/* for [caml_convert_signal_number] */
-#include <caml/signals.h>
-
-#undef CAML_INTERNALS
-
 #include <caml/mlvalues.h>
 #include <caml/memory.h>
 #include <caml/alloc.h>
 #include <caml/unixsupport.h>
 #include <caml/fail.h>
 
+/* for [caml_convert_signal_number]; must come after public caml headers */
+#include <caml/signals.h>
+
 #include <errno.h>
 
 #if defined(__APPLE__)