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

GCC 13 on Alpine 3.20 (musl) has a bug where atomic_wait.h uses
syscall() without including <unistd.h>. Adding it to the preinclude
header fixes the musl builds without affecting glibc.

Author: jonatansberg

src/gcc-preinclude.h

@@ -2,6 +2,8 @@
 
 #if defined(__linux__)
 
+#include <unistd.h>
+
 #define _GNU_SOURCE
 #include <features.h>
 #undef _GNU_SOURCE