mallinfo2: include malloc header even if mallinfo undetected

It may be the case that for some reason, -Werror deprecated
for instance, that mallinfo isn't detected. In this case the
malloc.h headers won't be included which defined the mallinfo2
function and its structure.

Re-organise so that either function pulls in the header.

sql/sql_test.cc

@@ -27,11 +27,13 @@
 #include "my_json_writer.h"
 #include <hash.h>
 #include <thr_alarm.h>
-#if defined(HAVE_MALLINFO) && defined(HAVE_MALLOC_H)
+#if defined(HAVE_MALLINFO) || defined(HAVE_MALLINFO2)
+#if defined(HAVE_MALLOC_H)
 #include <malloc.h>
-#elif defined(HAVE_MALLINFO) && defined(HAVE_SYS_MALLOC_H)
+#elif defined(HAVE_SYS_MALLOC_H)
 #include <sys/malloc.h>
 #endif
+#endif
 
 #ifdef HAVE_EVENT_SCHEDULER
 #include "events.h"