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

@@ -26,5 +26,8 @@ __asm__(".symver pow,pow@GLIBC_2.17");
 __asm__(".symver fcntl64,fcntl@GLIBC_2.17");
 #endif
 
+#else
+// musl libc: pull in syscall() for GCC 13's atomic_wait.h
+#include <unistd.h>
 #endif
 #endif