sys/cdefs.h: add definition of __attribute_alloc_size__

Commit cee0b058fa0b4501b289a2da365182d60314d746 ("add aligned_alloc
required for latest gcc libstdc++") added the prototype of
aligned_alloc() to <stdlib.h>. This prototype contains
'__attribute_alloc_size__ ((2))', but this is not defined anywhere in
uClibc-ng.

This commit addresses that by adding the relevant definition in
<sys/cdefs.h>, borrowed from glibc.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

include/sys/cdefs.h

@@ -222,6 +222,15 @@
 # define __attribute_malloc__ /* Ignore */
 #endif
 
+/* Tell the compiler which arguments to an allocation function
+   indicate the size of the allocation.  */
+#if __GNUC_PREREQ (4, 3)
+# define __attribute_alloc_size__(params) \
+  __attribute__ ((__alloc_size__ params))
+#else
+# define __attribute_alloc_size__(params) /* Ignore.  */
+#endif
+
 /* At some point during the gcc 2.96 development the `pure' attribute
    for functions was introduced.  We don't want to use it unconditionally
    (although this would be possible) since it generates warnings.  */