libcompat: Do not redeclare sys_siglist when the system does so

This can cause issues if the declaration is not exactly the same, and
declaring it when the system already does is bogus no matter what.

Signed-off-by: Guillem Jover <guillem@debian.org>

debian/changelog

@@ -3,6 +3,9 @@ dpkg (1.18.23) UNRELEASED; urgency=medium
   * Handle unmatched arch-qualified virtual packages in dpkg-genbuildinfo,
     instead of letting perl die. Closes: #849944
   * Declare .buildinfo format as stable with version 1.0.
+  * Portability:
+    - Do not redeclare sys_siglist in libcompat when the system does so.
+      Thanks to Thomas Klausner <wiz@NetBSD.org>.
   * Perl modules:
     - Do not special case EM_SPARC32PLUS for NetBSD in Dpkg::Shlibs::Objdump,
       the code has been fixed in NetBSD as that situation could not happen.

lib/compat/strsignal.c

@@ -60,7 +60,6 @@ const char *const sys_siglist[] = {
 #  define NSIG 32
 # endif
 # define COMPAT_NSIGLIST NSIG
-extern const char *const sys_siglist[];
 #endif
 
 const char *