Add the posix_memalign(3) function to bionic

The posix_memalign(3) function is very similar to the traditional
memalign(3) function, but with better error reporting and a guarantee
that the memory it allocates can be freed.  In bionic, memalign(3)
allocated memory can be freed, so posix_memalign(3) is just a wrapper
around memalign(3).

Change-Id: I62ee908aa5ba6b887d8446a00d8298d080a6a299
diff --git a/libc/include/stdlib.h b/libc/include/stdlib.h
index 5dc8a87..e5caadd 100644
--- a/libc/include/stdlib.h
+++ b/libc/include/stdlib.h
@@ -67,6 +67,8 @@
 extern unsigned long long strtoull(const char *, char **, int);
 extern double strtod(const char *nptr, char **endptr);
 
+extern int posix_memalign(void **memptr, size_t alignment, size_t size);
+
 static __inline__ float strtof(const char *nptr, char **endptr)
 {
     return (float)strtod(nptr, endptr);