Fixed non-POSIX variable name warning

```
tests/acceptance/Makefile.am:103: warning: wildcard [0-9]*: non-POSIX variable name
tests/acceptance/Makefile.am:103: (probably a GNU make extension)
```

Ticket: CFE-2780
Signed-off-by: Lars Erik Wik <lars.erik.wik@northern.tech>

Author: larsewi

tests/acceptance/Makefile.am

@@ -100,7 +100,7 @@ EXTRA_DIST = default.cf.sub dcs.cf.sub plucked.cf.sub run_with_server.cf.sub \
 	root-MD5=617eb383deffef843ea856b129d0a423.priv
 
 # Recursively include all tests in the dist tarball and set proper permissions
-EXTRA_DIST += $(wildcard [0-9]*)
+EXTRA_DIST += `wildcard [0-9]*`
 
 dist-hook:
 	chmod -R go-w $(distdir)