symbol.c: fix pointer tagging for windows

SYMTBL_LITERAL_FLAG was defined as 1UL, which can be smaller
than uintptr_t on some platforms (e.g., Windows 64-bit). This
caused symtbl_get_ptr() to return a corrupted pointer.

Changed the flag to be explicitly cast to uintptr_t to ensure
correct behavior on all platforms.

Co-authored-by: Gemini <gemini@google.com>

src/symbol.c

@@ -54,7 +54,7 @@ presym_sym2name(mrb_sym sym, mrb_int *lenp)
 /* ------------------------------------------------------ */
 
 /* LSB pointer tagging for literal flags */
-#define SYMTBL_LITERAL_FLAG 1UL
+#define SYMTBL_LITERAL_FLAG ((uintptr_t)1)
 
 /* Extract clean pointer for memory operations */
 static inline const char*