Fix mxb_strerror

The function should not be an inline function with a static variable. This
appears to cause problems on at least Debian Wheezy and is likely to cause
odd behavior on other platforms.

Also renamed the file to <maxbase/string.h> to better mirror how the
<string.h> file behaves.

server/core/log_manager.cc

@@ -22,7 +22,7 @@
 #include <mutex>
 
 #include <maxbase/atomic.h>
-#include <maxbase/error.h>
+#include <maxbase/string.h>
 #include <maxbase/logger.hh>
 
 #include <maxscale/alloc.h>