guix: adjust RISC-V __has_include() patch to work with GCC 10

The actual macro is __has_include(), not __has_include__(), using the
later would result in build failures when using GCC 10. i.e:
```bash
../sysdeps/unix/sysv/linux/riscv/flush-icache.c:24:5: warning: "__has_include__" is not defined, evaluates to 0 [-Wundef]
   24 | #if __has_include__ (<asm/syscalls.h>)
```

Looks like at least someone else has run into the same thing, see:
http://lists.busybox.net/pipermail/buildroot/2020-July/590376.html.

See also:
https://gcc.gnu.org/onlinedocs/cpp/_005f_005fhas_005finclude.html
https://clang.llvm.org/docs/LanguageExtensions.html#has-include

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