swanctl: Add missing extern for swanctl_dir variable in header

This clearly never was correct, but didn't cause problems so far.
However, GCC 10 will default to `-fno-common` instead of
`-fcommon` (https://gcc.gnu.org/PR85678), so compilation there fails
with something like:

```
libtool: link: gcc ... -o .libs/swanctl ...
ld: commands/load_authorities.o:strongswan/src/swanctl/./swanctl.h:33:
  multiple definition of `swanctl_dir'; commands/load_all.o:strongswan/src/swanctl/./swanctl.h:33: first defined here
```

Fixes: 501bd53a6c ("swanctl: Make credential directories relative to swanctl.conf")
Closes strongswan/strongswan#163.

src/swanctl/swanctl.h

@@ -30,7 +30,7 @@
 /**
  * Base directory for credentials and config
  */
-char *swanctl_dir;
+extern char *swanctl_dir;
 
 /**
  * Configuration file for connections, etc.