stdlib.h: Remove guards around aligned_alloc.

While FreeBSD and glibc feature-guard it, they also feature-guard
a lot of other things that we don't, and musl does not guard it,
so it seems more than safe enough to leave it unguarded.

Fixes compilation errors with GCC 11. (The other possible solution
was including features.h in more places, but this seems simpler.)

headers/posix/stdlib.h

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2021 Haiku, Inc. All Rights Reserved.
+ * Copyright 2002-2021 Haiku, Inc. All rights reserved.
  * Distributed under the terms of the MIT License.
  */
 #ifndef _STDLIB_H_
@@ -49,9 +49,7 @@ extern void		*calloc(size_t numElements, size_t size);
 extern void		free(void *pointer);
 extern void		*malloc(size_t size);
 extern int		posix_memalign(void **_pointer, size_t alignment, size_t size);
-#ifdef _ISOC11_SOURCE
 extern void 	*aligned_alloc(size_t alignment, size_t size) _ALIGNED_BY_ARG(1);
-#endif
 extern void		*realloc(void *oldPointer, size_t newSize);
 
 /* process termination */