plugins: Include trace/mem.h in api.c

Code movement in an upcoming patch will show that this file
was implicitly depending on trace/mem.h being included beforehand.

Tested-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reported-by: Alex Bennée <alex.bennee@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>

plugins/api.c

@@ -46,6 +46,7 @@
 #include "qemu/plugin-memory.h"
 #include "hw/boards.h"
 #endif
+#include "trace/mem.h"
 
 /* Uninstall and Reset handlers */