util: fix leak in getstyle's makeThemePack (Coverity #50164)

As pointed by Coverity, the function is creating a temporary array with
the list of keys of the Style dictionnary, but it was not freed in the end.

Signed-off-by: Christophe CURIS <christophe.curis@free.fr>

util/getstyle.c

@@ -279,6 +279,7 @@ static void makeThemePack(WMPropList * style, const char *themeName)
 			}
 		}
 	}
+	WMReleasePropList(keys);
 }
 
 int main(int argc, char **argv)