Fix scanTuple undeclared identifier 'scanf' (#24759)

Without this fix, trying to use `scanTuple` in a generic proc imported
from a different module fails to compile (`undeclared identifier:
'scanf'`):

```nim
# module.nim
import std/strscans

proc scan*[T](s: string): (bool, string) =
  s.scanTuple("$+")
```

```nim
# main.nim
import ./module
 
echo scan[int]("foo")
```

Workaround is to `export scanf` in `module.nim` or `import std/strscans`
in `main.nim`.

tests/stdlib/mstrscans_undecl_scanf.nim

@@ -0,0 +1,4 @@
+import std/strscans
+
+proc scan*[T](s: string): (bool, string) =
+  s.scanTuple("$+")

tests/stdlib/tstrscans_undecl_scanf.nim

@@ -0,0 +1,3 @@
+import std/assertions
+import ./mstrscans_undecl_scanf
+doAssert scan[int]("foo") == (true, "foo")