libcompat: Add missing <errno.h> include

Warned-by: gcc on AIX

debian/changelog

@@ -57,6 +57,7 @@ dpkg (1.18.11) UNRELEASED; urgency=medium
       ar_hdr struct, as the ar format is not standardized and does vary
       across systems, for example on AIX.
     - Add <sys/sysmacros.h> on AIX for major() and minor().
+    - Add missing <errno.h> in libcompat.
   * Perl modules:
     - Obsolete Source-Version substvar in Dpkg::Substvars by emitting errors.
     - Rework keyring hooks in Dpkg::Vendor. Deprecate the keyrings hook, and

lib/compat/strerror.c

@@ -19,6 +19,7 @@
 
 #include <config.h>
 
+#include <errno.h>
 #include <stdio.h>
 #include <gettext.h>