asn1: Include <errno.h> before using/checking bits from it

Commit 232c936ea3 added a fallback ENOTSUP
definition. The check itself and the fallback definition rely on
<errno.h> to be included already, so in case it was not (depending on
the platform) then there will be a wrong definition of ENOTSUP.

Instead of rely on <errno.h> to be included before including a
generated header from this tool, include it manually. This way both
the ENOTSUP check and its fallback definition (if ever) will always
work.

lib/asn1/gen.c

@@ -191,6 +191,7 @@ init_generate (const char *filename, const char *base)
     fprintf (headerfile,
 	     "#include <stddef.h>\n"
 	     "#include <stdint.h>\n"
+	     "#include <errno.h>\n"
 	     "#include <time.h>\n\n");
     fprintf (headerfile,
 	     "#ifndef __asn1_common_definitions__\n"

lib/asn1/template.c

@@ -38,6 +38,7 @@
 #include <vis.h>
 #include <vis-extras.h>
 #include <heimbase.h>
+#include <errno.h>
 
 #ifndef ENOTSUP
 /* Very old MSVC CRTs don't have ENOTSUP */