Fix astyle indentation of #include inside #ifdef in unit-items-cpp17.cpp

This repo's astyle style keeps preprocessor directives at column 0
even inside #ifdef blocks. The new tests/src/unit-items-cpp17.cpp
had its #include <map>/#include <string> indented, which made the
'check' CI job's amalgamation/formatting diff non-empty and failed
the aggregate check.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

tests/src/unit-items-cpp17.cpp

@@ -17,8 +17,8 @@
 using nlohmann::json;
 
 #ifdef JSON_HAS_CPP_17
-    #include <map>
-    #include <string>
+#include <map>
+#include <string>
 
 TEST_CASE("items()")
 {