Fix cram_bambam.c once more.

Fix silly mistake in 4df5bbd.  io_lib/os.h was included so it could
define CRAM_IO_CUSTOM_BUFFERING, so obviously it has to be before the
ifdef.

Author: jkbonfield

io_lib/cram_bambam.c

@@ -60,11 +60,12 @@
 #include "io_lib_config.h"
 #endif
 
+#include "io_lib/os.h"
+
 #if !defined(CRAM_IO_CUSTOM_BUFFERING)
 // not used
 #else
 
-#include "io_lib/os.h"
 #include "io_lib/cram_bambam.h"
 
 //-----------------------------------------------------------------------------