Fixed Alpine/musl builds: include unistd.h in gcc-preinclude

GCC 13 on Alpine (musl) has a bug where atomic_wait.h uses syscall()
without including <unistd.h>. On glibc this isn't needed because
_GNU_SOURCE pulls it in, so the include is conditional on musl only.

Author: jonatansberg

src/gcc-preinclude.h

@@ -4,9 +4,10 @@
 
 #define _GNU_SOURCE
 #include <features.h>
+#include <unistd.h>
 #undef _GNU_SOURCE
 
-#if defined(__USE_GNU)
+#if defined(__GLIBC__)
 
 #if defined(__x86_64__)
 __asm__(".symver memcpy,memcpy@GLIBC_2.2.5");