llama-mmap: fix missing include (#11796)

Technically the fixed width types come only from iostream and
cstdint/stdint.h headers. memory and vector headers should not provide
these. In GCC 15 the headers are cleaned up and you require the proper
header cstdint.

src/llama-mmap.h:26:5: error: ‘uint32_t’ does not name a type
   26 |     uint32_t read_u32() const;
      |     ^~~~~~~~

src/llama-mmap.h

@@ -1,5 +1,6 @@
 #pragma once
 
+#include <cstdint>
 #include <memory>
 #include <vector>