curl_sasl: declare mechtable static

struct mechtable is only used locally here. It can be declared static.

lib/curl_sasl.c

@@ -50,7 +50,7 @@
 #include "memdebug.h"
 
 /* Supported mechanisms */
-const struct {
+static const struct {
   const char   *name;  /* Name */
   size_t        len;   /* Name length */
   unsigned int  bit;   /* Flag bit */