Fix 'use of undeclared SIGSEGV' on Linux if malloc redirection

(fix of commit 54d8c74)

Issue #588 (bdwgc).

* os_dep.c [MPROTECT_VDB && DARWIN || OPENBSD]: Include signal.h.
* os_dep.c [NEED_FIND_LIMIT || UNIX_LIKE && !NO_DEBUGGING
|| USE_PROC_FOR_LIBRARIES && THREADS || WRAP_MARK_SOME
&& NO_SEH_AVAILABLE]: Likewise.
* os_dep.c [MPROTECT_VDB && !MSWINCE]: Do not include signal.h
if MSWIN32.

os_dep.c

@@ -17,8 +17,8 @@
 
 #include "private/gc_priv.h"
 
-#if (defined(MPROTECT_VDB) && !defined(DARWIN) && !defined(MSWINCE)) \
-    || defined(GC_SOLARIS_THREADS)
+#if (defined(MPROTECT_VDB) && !defined(MSWIN32) && !defined(MSWINCE)) \
+    || defined(GC_SOLARIS_THREADS) || defined(OPENBSD)
 # include <signal.h>
 #endif
 
@@ -873,6 +873,8 @@ GC_INNER void GC_setpagesize(void)
     || (defined(USE_PROC_FOR_LIBRARIES) && defined(THREADS)) \
     || (defined(WRAP_MARK_SOME) && defined(NO_SEH_AVAILABLE))
 
+#   include <signal.h>
+
 #   ifdef USE_SEGV_SIGACT
 #     ifndef OPENBSD
         static struct sigaction old_segv_act;