Include stddef.h for size_t

#include "alpi.h"

int main(int argc, char *argv[]) { }

In file included from sample.c:1:
alpi.h:108:52: error: unknown type name ‘size_t’
  108 | int alpi_info_get(alpi_info_t query, char *buffer, size_t max_length, int *length);
      |                                                    ^~~~~~
alpi.h:13:1: note: ‘size_t’ is defined in header ‘<stddef.h>’; this is probably fixable by adding ‘#include <stddef.h>’
   12 | #include <stdint.h>
  +++ |+#include <stddef.h>
   13 |