Include util/strencodings.h which is required for IsSpace(...)

src/test/getarg_tests.cpp

@@ -2,6 +2,7 @@
 // Distributed under the MIT software license, see the accompanying
 // file COPYING or http://www.opensource.org/licenses/mit-license.php.
 
+#include <util/strencodings.h>
 #include <util/system.h>
 #include <test/test_bitcoin.h>