[FIX] use mixin for strscans.scanp (#17371)

lib/pure/strscans.nim

@@ -577,6 +577,7 @@ macro scanp*(input, idx: typed; pattern: varargs[untyped]): bool =
     of nnkCallKinds:
       # *{'A'..'Z'} !! s.add(!_)
       template buildWhile(input, idx, init, cond, action): untyped =
+        mixin hasNxt
         while hasNxt(input, idx):
           init
           if not cond: break
@@ -688,4 +689,4 @@ macro scanp*(input, idx: typed; pattern: varargs[untyped]): bool =
   result.add toIfChain(conds, idx, res, 0)
   result.add res
   when defined(debugScanp):
-    echo repr result
+    echo repr result