meson: fix detection of libcryptsetup functions

meson: fix detection of libcryptsetup functions

Meson would generate the following compile test:

  #define crypt_set_metadata_size meson_disable_define_of_crypt_set_metadata_size

  #include <limits.h>
  #undef crypt_set_metadata_size

  #ifdef __cplusplus
  extern "C"
  #endif
  char crypt_set_metadata_size (void);

  #if defined __stub_crypt_set_metadata_size || defined __stub___crypt_set_metadata_size
  fail fail fail this function is not going to work
  #endif

  int main(void) {
    return crypt_set_metadata_size ();
  }

This works fine when the identifier being queried is an actual function. But
crypt_token_max() is an inline function, so getting the address would fail,
leading to a false negative result. Complation would fail because the function
would be defined twice.

With this patch, the check is changed to include the header:

  #include <libcryptsetup.h>
  #include <limits.h>

  #if defined __stub_crypt_set_metadata_size || defined __stub___crypt_set_metadata_size
  fail fail fail this function is not going to work
  #endif

  int main(void) {
    void *a = (void*) &crypt_set_metadata_size;
    long long b = (long long) a;
    return (int) b;
  }

which seems to work correctly.